Sound problem on the net in Linux

I use Ubuntu 8.04, 32-bit version, with Regular update.
During the viewing of videos net does not have sound, just images.I mention that if you download to your PC, these videos have sound. I have this problem to sound only when it comes to the net, the PC programs were sound.
Use the Firefox browser.Because I have not been able to solve the problem of will require help.

Thank you!

I got the same problem with Linux..and couldn't find the solution. so I solved it by changing my operating system.

for trying ubuntu you need to install libflashsupport .