I'm the one who set up stretch77 with Ubuntu, so I'll describe the issue in a little more detail. At first it seemed to me to be the normal issue where flashplugin-nonfree grabs the sound and won't let go until the browser is closed. With that issue, I've also noticed the inverse, where is you are playing sound, lets say in Rhythmbox, then flash won't play sound until the other program is closed.
Having warned stretch77 of this, he booted his laptop, and immediately opened Firefox, and only Firefox. However, he still cannot get any sound from flash videos, though all other programs work just fine. Due to this, his symptoms seem a little different than the normal problems, but I could be wrong.
To stretch77 I'll say this: Go ahead and install the package that Happy_Man mentioned, using the first Terminal line of code below. Then browse the web as before, and see if it fixes your problem without giving you stability issues. If it does not, use the second of the two lines to remove it. Do keep us posted on how it goes.
sudo apt-get install libflashsupport
sudo apt-get remove libflashsupport