I am not very knowledable about plugins or 'addons' or extenstions. Plenty of hard drive space (using an SSD boot drive btw). I scanned pc with ESET online antivirus, no viruses, Malewarebytes Pro, no issues.

Even tried to restore computer to an earlier setting ( I only went two days ago, could go farther back I suppose).When I reverted back, the problem went away for a few hours but then came right back. I have tried many things include 'reset firefox', clear cache, changing to an earlier version of Flash (11.3). Windows 7 64 bit system, Intel Core I7 Processor. Running latest version of Firefox (26.0) and Adobe Flash (11,9,900,170 installed). Once the sound stops however, if I open a new youtube video in another window, it will sometimes cause an 'An error occured. If I restart the computer, or firefox only sometimes (mostly need to restart cpu though) then videos will play again with sound. I verified this wasn't a hardware issue with my headphones by opening the Speaker Icon Mixer window, and acutally was watching it at the moment the sound stopped and saw the green bar disappear exactly when the sound stopped.

Oddly enough, usually the video keeps going even thought the audio has stopped. Then after a few minutes or after having a few tabs open, all of a sudden the audio stops. When I open the speaker icon mixer window in the bottom right, I see green bar moving with the sound on the 'speakers' mixer and 'Adobe Flash Player' mixer simultaneously. Right now I am using headphones (1/8' or 3.5mm) through the back of the computer and when I turn on the computer, at first I can hear sound from videos. What's been happening is weird audio issues, especially youtube related though not only (other web video content has been messed up, but mostly youtube).

Realplayer 16.0.3.51 not playing