Ubuntu Jaunty Jackalope and simon are not exactly best friends. If you tried it, you most likely experienced some very strange sound issues.
simon hangs, something else hangs, everything hangs...
I installed Kubuntu 9.04 today and had the very same issues.
Turns out the problem is a pretty old portaudio version.
The current snapshot fixed all those problems for me. You can get it on the portaudio website. Download the pa_snapshot.tgz, compile and install.
I haven't yet found a pre-packaged debian package to spare you the compiling part. If you know one, please let me know in the comments.
Also, it worked for me but obviously YMMV. Still issues? Again: let me know in the comments.