I have a fully up to date system and have just noticed in Firefox that the sound reverts to zero after you play a video. So if I click a Youtube video sound works OK, click on another video and no sound, I can select volume in widget but not Youtube ,and increase it although also not with multimedia keys. All other applications sound is working fine including multimedia keys. A Google shows a similar problem in the past with Firefox but with volume being reset to 100% not 0. No mention of anyone else with this problem. I tried Youtube with Konqueror and all works as normal.