I know you're using the chip for gfx as well, but couldn't you spare a few pins for outputting PWM audio? It'd be stupid easy to have a bunch of preprogrammed waveforms (including a random noise generator) and the ability to switch between those and samples at runtime. That way you'd have a couple of GB-wav-like (yes I know it's PCM) channels, which would be really fun to write for.
Failing that, you could always add a dedicated PSG chip, keep everyone happy.