I tend to crawl the app store when I'm bored and just came across a few interesting apps.
Glitch Machine: A generative music synth (The video shows it off a bit.)
http://madgarden.net/apps/glitch-machine/
Famicompose: An MML composer and player (The author also has, among others, 3 other fami related apps, piano, guitar, and sound.)
http://www.geocities.jp/gamedokan/
Bonus! There is also has a nes sound tester in ROM format.
p.s. I am not associated with either person.