I came across an interesting problem today.
I have just reinstalled my full system of Linux Mint 13 MATE/Cinnamon and all is working well bar one fault – I tried to play a YouTube video and it played back at at least twice the speed. Naturally audio was non-existent too.
My first suspect was the browser, so I tried running a local MP4 file in Gnome Movie Player and ran into the same problem which eliminated the browser or any plugins.
The culprit, as I discovered is Pulse Audio.
I have Pulse Audio Volume Control installed and found the solution straight off – I went into the Configuration tab and set my soundcard configuration to "off".
All videos now work normally as does sound.