If you want tons of sound effects that are free for educational purposes, go to this BBC site (BBC Sound effects):
http://bbcsfx.acropolis.org.uk
You can click on the icon to play the sound, but sometimes, you will want to play the sound as your program runs (when there is a collision or a winner for example). The name of the sound clip is a command that plays the sound. Say you have a sound named cheers on your page (visible or not). You can type cheers in the Command Centre, or include it as an instruction in a procedure.
Here is an example from our Get Set, Go sample. When the race starts the sound (cheers) begins.
to StartRace forever [cheers] everyone [clickon] end