I have a "William Stuart Systems Input/Output Port and Music Synthesiser for ZX81 and Spectrum" connected to my ZX81 and that uses AY-3-8910, see page 7 in http://df.lth.se.orbin.se/~mikaelb/sinc ... l_zx81.pdf.
I think the same chip is used in "ZON X-81 Programmable Sound Generator", see http://www.zx81stuff.org.uk/zx81/genera ... ONX81.html.
Using this project with ATmega8 one could probably build sound cards for ZX81. Unfortunately there doesn't seem to be any source code, but only hex-files.
The AY and YM chips will run out one day, so the idea is good.
As for the compatibility - obviously I'd have to listen to some my song from it, to really judge.
Anyway, the idea itself is OK.