I came across a strange problem. I had no sound in flash videos. All the videos on Youtube were without sound. I am using Ubuntu as distribution with Firefox as browser. After some hours of researching I came across this fix:
  1. sudo apt-get install alsa-oss
  2. sudo gedit /etc/firefox/firefoxrc.
  3. Change the line FIREFOX_DSP="none" to FIREFOX_DSP="aoss".
  4. Save and quit
  5. go to System - Preferences - Sessions - Startup programs and add: ln -s /tmp/.esd-1000 /tmp/.esd
  6. Reboot your computer

Should work now.