Hi there " outofthejar"!
The built-in playback controls for both version 2 and version
3 include an audio-mute button right on the control. That button
will mute all audio during the movie until it is clicked "On"
again.
If you feel that you need a mute control for each individual
slide, I suggest you visit Paul Dewhurst's Captivate Tips web-site
and get his widget (SWF) for a mute control. Paul is a Captivate
Community Expert who lives in Liverpool, England.
Paul's site
is located at this link
, and you can follow his internal links from there to find what
you want. Good luck!