I've been playing with snd_play.prg on Atari St which has a decent large collection of music for this program. Lot of it isn't so great, but I'm just browsing now, maybe there are some gems in here. I'm also aware of mod files for Amiga, and there's a large archive of these. What other cores have interesting, non-mainstream collections of music. Mostly interested in non-mp3/wav, non-midi music. The quirkier and retro-ier the better.
Here are obvious links I just know of. But I'm not sure, are there music archives for the Japanese cores or the TRS-80 varients? I googled for TI-99/4a music, and didn't find all that much.
Atari ST: http://sndh.atari.org/
Amiga Mod: https://modarchive.org/
Atari 8-bit/Pokey: https://asma.atari.org/
C64/SID: https://www.hvsc.c64.org/
Spectrum: https://zxart.ee/eng/music/mainpage/