I found the hint to change the default sound for iCal alarms, I haven't tried it yet, I'm going to use Terminal to be able to edit the plist:
http://www.macworld.com/article/132412/2008/03/105icalalarm.html
What should I do if I want no sound instead of some other sound?
http://www.macworld.com/article/132412/2008/03/105icalalarm.html
What should I do if I want no sound instead of some other sound?