SuperCollider
SuperCollider is a realtime software synthesizer. Rohan Drape has written wrappers to OpenSoundControl system and SuperCollider. With these packages you are able to control the SuperCollider server by Haskell instead of SuperCollider's built-in language and you can perform interactive SuperCollider sessions from within GHCi.
You can use SuperCollider for performing music using Haskore and http://darcs.haskell.org/haskore-supercollider/ . Watch an example video.
There is also on-going effort to use SuperCollider and Haskell for speech synthesis.