PMD98 (Professional Music Driver). One in a series of music drivers done by Masahiro "KAJA" Kajiwara (or Kajihara, not sure of the romanization here). Like the name says, that's for PC-98 computers (there's MMD98 for MIDI files, PMD88 for PC-88, PMD68 for X68000, PMDIBM for, well, PCs (I've seen this used in some old DOS hentai games for instance, Seasons of Sakura or Three Sister's Story), probably more.
It was used in a bunch of games (I'd guess more than 200, but there's no complete list so it's possible the number's higher). And as it was made freeware sometime ago, there are also some fan works in this as well (though they mostly are just convertions of other games).
Here's the website of the author: http://www5.airnet.ne.jp/kajapon/
IIRC there's a short list of his works somewhere there.

Anyway, back to PMD98, there are three music subformats (though 90% of the stuff is in the first one) and a few sample pack formats:

.M - that's the standard music file. OPN compatible (so, 3 FM channels + 3 PSG channels, mono)
.M2 - some games have additionaly this. OPNA compatible (6 FM channels + 3 PSG channels + Percussion samples + ADPCM sample channel, stereo)
.MZ - don't recall having any of those. Seem to use the ADPCM sample channel to mix up to 8 sound channels.

Sample pack formats:

.PZI
.PPS
.PPC
.P86

Not sure what the differences are here.

Anyway, the in_fmpmd plugin is smart enough to just load any matching found sample pack. With .M files there's no problem, since they don't have any, but when you try to play .M2 file without the coresponding sample pack, the player won't even load the file. I have quite large collection of the sample packs, so all files should play, but it's kinda messy there. :)

On to FMP Sound System. It's another music driver, this time PC-98 only. Done by Guu, wasn't as widely used (only a bunch of games, most of the stuff I have are fanmade conversions of other games). There was also a series of scene-like musicdiscs (FMP Music Disk), but all I could find about them were some screenshots from the intro/music selector (that's very Japanese - make a kicking program, and just keep it to yourself/you circle of friends). AFAIK those were sold on some conventions, so that's prolly the main reason there are nowhere to be found. Once again, three music subformats:

.OPI - standard, OPN compatible.
.OVI - enhanced, OPNA compatible.
.OZI - more enhanced, OPNA with mixed samples.

As for sample packs, .PVI and .PZI seem to be used.