If you like to listen to game music on infinite loop, here is a handy guide to how to (and whether you can) loop the music you get.
Generally speaking, soundtrack releases (on CDs or MP3 sets) are not made to be infinitely loopable, but instead just contain the intro (if applicable), one or two loops, and a fadeout. So you can't just let them play indefinitely to set the mood. But frequently, emulated sound formats take the actual game data and are able to loop that.
The following guide summarizes the possibility and methods of looping game music from various systems.
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NOTE1: You can always loop stuff by re-processing the recording in a program like Audacity (record from your speakers if necessary to produce a raw WAV/MP3 that can be used with Audacity) to find the a place where you can seamlessly loop the track, and record your own hour-long (or however-long) looped version. It might even be possible to make a looping OGG by yourself. I call this the "brute force" method -- a method of last resort.
NOTE2: I make no guarantees that any of the tagging will be correct.
NOTE3: Not all downloads of some of the more recent systems' games contain the streams (e.g. XA files). If you get CvSotN and find it only contains two tracks, don't be surprised; it's just missing the streams (which is most of the soundtrack). This is because the streams are kinda huge.
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NES games: most games have been ripped into NSF files, which are multi-track files that contain their soundtracks. They can be looped with appropriate foobar2000/Winamp plugins or players (such as VirtuaNSF).
Sega Master System, Game Gear, GG, Genesis games: most games have been ripped into VGM or VGZ files, which are single-track files that can be looped with appropriate foobar2000/Winamp plugins. VGZ is basically just zipped VGM. Some Genesis games also have GYM files, which are single-track files that can be played but not looped with appropriate foobar2000/Winamp plugins.
GB games: most games have been ripped into GBS files, which are multi-track files that contain their soundtracks; they can be looped with appropriate foobar2000/Winamp plugins or players. Sometimes, GB ROM files are also available that simply compile the soundtrack into a format that can be played on a GB emulator.
SNES games: most games have been ripped into SPC files, which are single-track files that can be looped with appropriate foobar2000/Winamp plugins or players (e.g. WinSPC). Some sites offer RSN files, which are renamed RAR files containing the SPCs.
MSX games: at least some games (I don't know the full extent) have been ripped into KSS files, which are multi-track files that can be looped with appropriate foobar2000/Winamp plugins. They usually come with a M3U playlist which organizes the files in proper order and gives them appropriate timing; otherwise they just loop infinitely.
Turbografx/PC Engine games: at least some games have been ripped into HES files, which are multi-track files that contain their soundtracks; I think they can be looped with appropriate foobar2000/Winamp plugins or players.
PS1 games: most games have been ripped into PSF/MINIPSF+PSFLIB files and/or XA files. PSF files are single-track and loopable with appropriate foobar2000/Winamp plugins. MINIPSF files are single-track, and actually pointers to a single PSFLIB file with the music data, which should be placed in the same directory. XA files are streams and cannot be looped by themselves (that I know of), though it may be possible to make long playlists of them that combine intros and loops, depending on how they are stored.
SPECIFIC GAMES:
* Castlevania: Symphony of the Night: the XA streams can be strung together in playlists to create near-infinite looping for most tracks for which looping is relevant, but a few tracks cannot be properly looped because one XA file may contain multiple tracks worth of data.
PS2 games: most games have been ripped into PSF2/MINIPSF2+PSF2LIB files and/or XA, SVAG, MIB, and/or SS2 files. PSF2 files are single-track and loopable with appropriate foobar2000/Winamp plugins. MINIPSF2 files are single-track, and actually pointers to a single PSF2LIB file with the music data, which should be placed in the same directory. XA files are streams and cannot be looped by themselves (that I know of), though it may be possible to make long playlists of them that combine intros and loops, depending on how they are stored. SVAG files and MIB files appear to be loopable and are probably streams. SS2 files appear to be linked to FMVs so I don't know how looping applies to them yet.
N64 games: most games have been ripped into USF/MINIUSF+USFLIB files. USF files are single-track and loopable with appropriate foobar2000/Winamp plugins. MINIUSF files are single-track, and actually pointers to a single USFLIB file with the music data, which should be placed in the same directory.
some arcade games that use Capcom Q-Sound: these games may have been ripped into QSF/MINIQSF+QSFLIB files. QSF files are single-track and loopable with appropriate foobar2000/Winamp plugins. MINIQSF files are single-track, and actually pointers to a single QSFLIB file with the music data, which should be placed in the same directory.
Sega Saturn: at least some games have been ripped into SSF/MINISSF+SSFLIB files. SSF files are single-track and loopable with appropriate foobar2000/Winamp plugins. MINISSF files are single-track, and actually pointers to a single SSFLIB file with the music data, which should be placed in the same directory.
Sega Dreamcast: at least some games have been ripped into DSF/MINIDSF+DSFLIB files. DSF files are single-track and loopable with appropriate foobar2000/Winamp plugins. MINIDSF files are single-track, and actually pointers to a single DSFLIB file with the music data, which should be placed in the same directory.
Game Boy Advance: most games have been ripped into GSF/MINIGSF+GSFLIB files. GSF files are single-track and loopable with appropriate foobar2000/Winamp plugins. MINIGSF files are single-track, and actually pointers to a single GSFLIB file with the music data, which should be placed in the same directory.
SPECIFIC GAMES:
* Uchuu no Stellvia - the soundtrack rip is missing the title screen track; which can be obtained by running rom2gsf on the game ROM itself.
* Scurge: Hive - this has not been ripped yet. Use the DS version of the soundtrack instead, though I think it's a little different.
* Juka and the Monophonic Menace - this has not been ripped yet. You can get the title screen theme by running rom2gsf on the game ROM.
NDS games: many games have been ripped into 2SF/MINI2SF+2SFLIB files and/or STRM files. PSF files are single-track and loopable with appropriate foobar2000/Winamp plugins. MINI2SF files are single-track, and actually pointers to a single 2SFLIB file with the music data, which should be placed in the same directory. STRM files are stream files and I am not yet sure how to loop them properly.
GameCube and Wii: at least some games have been ripped, into format such as HPS, SSM, FSB, DSP, and RSF. DSP files are single-track files that may be (but are not necessarily) natively loopable; a notable feature of them is that they sometimes contain separate stereo-left and stereo-right tracks, which are automatically combined (when in the same directory and either is opened) by your player. FSB and HPS files are single-track files that appear to be natively loopable from what little I've seen of them. SSM files are actually collections of multiple files that have to be extracted using a specialized program. RSF files do not appear to be natively loopable.
SPECIFIC GAMES:
* Mega Man Network Transmission's DSPs do not loop natively for some reason.
PC games: these soundtracks have a large number of formats, naturally.
* Note: some games just don't natively loop their soundtracks at all, period. For example: 100% Orange Juice, Eternal Daughter, Fortune Summoners. If you want, you could possibly create a loop yourself in some cases...
* MID: some players are able to handle MIDI looping data. I am not sure which ones.
* ORG: games that use this format (such as Cave Story) have individual tracks that can be opened up in the ORG Maker program and played there. Cave Story and Cave Story beta version also have standalone EXE files that contain the soundtracks wrapped in a player. There may be an ORG player plugin for foobar2000 but it does not seem functional.
* OGG, Falcom games: the OGG files are loopable when played using the program XMPlay.
* OGG/SLI, Edelweiss games: using an appropriate foobar2000 plugin, one can infinite-loop the tracks by playing the SLI files.
* Mutant Mudds: the entire soundtrack does not have intros, and thus each track can be easily looped by simply having a player repeat each track infinitely, with no gap.
* FSB files : you can extract MP3s out of them, but it seems the loop data is stuck inside the FSB file. I think. Not sure how to loop them. Not to be confused with FSB format for GC/Wii games.
* MOD, IT, S3M, XM (e.g. Lyle in Cube Sector): these can be played by the ModPlug Player and seem to be natively loopable, though I don't have much experience with them.
Comments
There is an old standalone player somewhere that can play RSN files natively. I forgot it's name.
Some plugins differ by whether they turn a multi-track file into multiple playlist entries on your player or use a single entry that "contains" multiple tracks. This may affect looping behavior.
There does exist a way to a GBS file into multiple GSF files.
I usually prefer to get plugins that control one filetype each, because I remember running across this one plugin that handled NSF, GBS, SPC, and a bunch of other common formats, but didn't offer the option to loop stuff infinitely. I forgot its name, though.
DPS SoundTouch plugin, however, is a really nifty tool for pitch-shifting, tempo-shifting, and speed-shifting music, though. So you should get it anyway.
* all looping-applicable tracks are loopable by virtue of being intro-less.
* OGG files can be obtained by running the game, THEN navigating to the game folder. OGG files will have appeared.
These OGGs do not contain loop data, according to XMPlay.
* most looping-applicable tracks are loopable by virtue of being intro-less.
* the following tracks seem to have hiccuping when looped seamlessly: mus_track_3, mus_track_9, mus_track_14, mus_track_17
These OGGs do not contain loop data, according to XMPlay.
The original version's music can be played using the La-Mulana Jukebox, which is a standalone program. It features chiptune and MIDI (with correct instrumentation) versions of all the tracks in the original game, including unused bonus tracks.
You can set it to loop infinitely.
Not all the tracks are accessible on startup. A password is needed to access the full soundtrack; the password is "zeus". Alternatively, you can rename your desired track in the program's files, so as to "trick" the player into playing the track you want to hear; if you do the latter, you'll need four instances of the player to cover the entire soundtrack.
Also, Recettear uses no-intro no-outro OGGs.
A few other tracks seem to consist of what would otherwise be natively-loopable tracks but with an added silence at the end. I think it should be a trivial job to remove said silence in Audacity.
I will confirm in a few days whether the CD version of the soundtrack is like this, but most likely it will be.
AT3 files are usually loopable, but for some reason, for Falcom's games, they aren't. In the case of Falcom, they do have a minor bit of outtro instead.
Meanwhile, OGGs with loop data can be renamed to extension LOGG which can then be played in foobar2000.