PlayStation Portable coder ethan, who developed a complete synthesis/sequencer
system for PSP console (http://dspmusic.org/psp/) (yes, a synthesis system
on the PSP ) has released a beta version of his sequencer PSPSeq 2.10 for Windows here:
Not the easiest control system in the world but fun/interesting enough to give it a try.
No source code, sorry...
Moreover http://dspmusic.org/ has lot of interesting info for pepole interested in
audio implementation on dsp..