I think you should post this question to some other section, you know, Hardware and Applications(Serious Stuff) come to mind.
By the way, there are TONS of audio related programs (shuold check Tim's Atari Midi Page) and quite a lo of hardware modifications and add ons (Video cards, Accelerators, and such).
Ah, there are also some other OSes on Atari, but NEVER *EVER* talk about WIN here
(although there has been an hardware card to emulate a 286 CPU and there also is a software DOS emulator). MiNT is a good multitasking Unix-like alternative OS, and there is Linux as well