I find this problem very interesting. Win7 RC running on an Ubuntu 9.04 host running 2.2.4, with Guest Additions installed. I have FIrefox installed with the appropriate Flash Installer added, and it works just fine on Youtube, at least for a day or two. At some point the Flash player will no longer deliver audio, and it plays videos at 2 or 3x speed. Quitting Firefox doesn't resolve it, nor does pausing the VM. A shutdown/restart of the guest restores all to normalcy. Pausing the guest does seem to extend the guest's viability in this regard, delaying the onset of the problem.
It doesn't seem to affect other apps, but it makes the Britain's Got Talent vids go extra fast. Anyone got ideas on this?