slipper,
Here's a workaround / quickfix....
First, you'll need to create another version of your video,
one without sound, and upload it to Youtube.
On your webpage, play that soundless video in an iframe.
Below the iframe, include a hotlink that says "Enable Sound".
Clicking that link will load the main video (the one with the sound)
into the iframe. Remember to use the Youtube autoplay parameter.
trish : - )