Sega Master System / Mark III / Game Gear
A Z80 ASM library for SN76489 background music with SFX support. Basically, this plays a (converted) VGM and, on top of that, can play another (converted) VGM, albeit restricted to using third square wave channel and/or noise channel only. You can create your SN76489 VGMs using Mod2PSG2, Deflemask or VGM Music Maker (no link). There may be other tools I never heard of.
(includes KiddEd ROM modifications)