Comment 4 for bug 151849

I do not have this problem (Ubuntu 7.10 64-bit), although I can't try it at this moment since Youtube is down. Try using the method here to install the newest version of Flash- 9.0 Update 3: http://ubuntuforums.org/showthread.php?p=3923465

And although this seems obvious, try making sure that sound does work in other applications (make sure everyone here is reporting the same exact bug). It is possible that System -> Preferences -> Sound Preferences, which has been known to be faulty, may have changed your sound settings. Navigate to your home directory (in Nautilus- the Ubuntu file manager), enable the viewing of hidden folders (View -> Check "Show Hidden Files" or simply Ctrl-H), and delete the file .asoundrc if it exists (This will remove any custom sound settings that you have).

More information is needed for the bug report.