If you want to generate sound by typing lists of numbers, check out csound
. Anything you write can be midi-controllable as long as your computer can render it in real-time. (If you're not so much into typing, there's a few dozen graphical front ends that might suit you -- I'd suggest a look at blue
, with a lisp-like language, for the CS students out there. Supercollider
, for people with macs. (Kidding. It also works, reluctantly, on other platforms.) PureData
if you prefer clicking over typing. Or, y'know, Buzz
, which afaik is free but not open source.