Amiga A500 as an electronic music synthesizer

[Hyperspeed]

You could try ParNet + parallel cable for linking the A500 and PC. Samba could be useful also.

Ethernet would be tricky but a parallel connection would be faster than serial - careful about making your own cable though as it's easy to fry stuff (it's happened before!).

Octamed is a good sound package for Amiga but there are many more.

If you had a beefier Amiga I would have suggested this TB303 emulator:
http://de4.aminet.net/misc/emu/TB303.lha

As Fatboy Slim might say "Everyone needs a 303!"

There are MIDI interfaces available for Amiga too and I think it was Coldcut and Aphrodite that used Amigas.

[Hyperspeed]

by Oliver:
and the results were most often akin to a flatulent jackhammer

The results you'd get from the TB303 emulator were more 'constipated elephant'...