As part of the development of the ZXodus][Engine, which underpins U3.5, I’ve built an audio engine. ColdSnap Audio is a cross-platform music engine for Z80-based computers with AY-3-8910 (and derivative) sounds chips (as used by the Mockingboard for the Apple ][). In fact a friend of mine is looking at doing a 6502 port for the Oric, which might eventually lead to an Apple ][ version. The engine comes in at around 1.8KiB of code. It features ADSR envelopes based on real instruments (brass, choir, flute, guitar, harp, marimba, organ, piano, and strings). Note tables are generated accurately based on the clock frequency of the AY chip in each machine. This ensures that when you listen to the U3.5 music on the real machine (or an emulation of it) it sounds the way it does on the SoundCloud site.
[Edit] ColdSnap Audio is now available to purchase separately: https://zx.itch.io/coldsnap