Attached is a simple example that shows you how to use events and a boolean valued variable to turn the music on and off. (Sorry about the choice of sounds - it gets annoying very quickly)
You'll need to vary it slightly so that you will also switch to the other song. (perhaps use two boolean valued variables).
For video tutorials on programming in Alice 2.2 check out my YouTube channel
http://www.youtube.com/user/drdave785
Also check out my Programming in Alice blog at
http://programminginalice.com