It could just be that the PulseAudio isn't working.
The Ubuntu 8.04 (a.k.a Hardy) has been having some trouble with PulseAudio and Flash 9.
You should install the libflashsupport package by going to the console and then running:
sudo apt-get install libflashsupport
After the install, you should restart the browser and try again.
Your problem is based on that the PulseAudio doesn't play audio in Flash 9, by default, so that's why you should do the thing above.
Try this an tell what happens