Did a little bit of searching, check this:
http://en.kioskea.net/forum/affich-19119-no-sound-from-web-browser

Appears this issue isn't uncommon, and the original poster shares your symptoms.
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Curious: have you tried removing all drivers, then rebooting? I keep thinking this has to do with Windows behaving funny specifically when trying to communicate with the sound device, yet....

This...