I use different flash for playing videos. Flash 10 on my youtube and flash 11 has to be used on videos posted by others on sites such as this, facebook etc. With those I try to go direct to my youtube, which reverts back to 10.

The reason I have done this is because 11 has some sort of bug connected with Mozilla which screws up playback, particularly sound, whereas reverting to flash 10 (which you can still download from certain sites) cures it.

It's damned annoying, but I'm not expert enough to sort out the issues with 11.

You can tell which flash is in use by right-clicking on any vid.

This is probably totally unrelated to your problem, but...