For what looks to be a very in-depth Smalltalk musical package, take a look at Siren (
http://fastlabinc.com/Siren/main.html). I haven't been able to get the supporting dll's loaded properly to try it out, but I think it even has a notation editor of some kind...
It's written for VisualWorks, not Squeak, but has an extensive manual which might give you some ideas for your project.
I have to keep reminding myself that the entire programming world is full of "examples" that don't work when you download them; it's not just a
Smalltalk phenomenon--but it IS frustrating nonetheless...
Rob