jMusic
I've taken a quick tour through a number of music composition languages. I've settled in on jMusic. First, it is all Java so I don't have to learn or relearn another programming language. It is extendible so it should be possible to add support for CSound score files, and it supports MIDI and JavaSound out of the box, so I can hear what I am doing as I go along, and finally, it is an active project with good documentation.