Sound was generally working fine on my Mandriva 2007 workstation, but sound in the Flash plugin was not. Here's how I debugged the problem and fixed it.

First, running Firefox from a terminal provided a great clue. To launch Firefox this way it may work to use Alt-F2 to have the run box appear, and then type "mozilla-firefox" and make sure "Run in terminal" is selected".

This should lots of ALSA-related errors, including "device not found".

By launching the Mandriva Control Center (Alt-F2 then 'mcc'), I was able to review the sound drivers in use. I navigated to "Hardware : Look and Configure Hardware" and then selected my soundcard below the "Soundcard" option. My soundcard was listed as "82801EB AC'97 Audio". From there, I could select "Run Config Tool", which provided a list of driver options. Only one of them was listed as "ALSA", so I switched the driver to that.

It didn't say I needed to reboot after that, but I believe I did.

I then installed 'alsamixergui' and ran it to raise all the volume levels and unmute them.

I then also used "Kmix" to unmute things.

After these steps, sound worked in Flash videos, and everywhere else, too!

By running Firefox once more from a terminal, I could see that no more ALSA-related errors were being generated.

If you have the same problem I hope this write-up has been helpful. I'm not an ALSA or sound expert so if these steps do n't help you, I doubt I could be off further help. Try asking on a Mandriva forum.

By contrast, I don't recall running into this problem on Ubuntu Linux. I know this worked at one point on Mandriva, and I'm not sure what made it quit working.