I have exactly the same problem and reported it here over one year ago. I have seen the issue surface again from time to time in discussions here but with no resolution.
I have been watching this for long enough that I have tried to determine which videos will not play the sound properly. It is definitely Not All of them. As close as I can tell, about 50% of all videos will not play the audio -just static. However, if I play any one of them again say a few days later - they may play properly. No apparent rule for which or when the sound will fail.
My work around is to install "YouTube Video Download and Convert" addon. If the YT standard player fails to play the sound you can choose a secondary player within the YT page. If I choose the WEBM format option, the video will most likely play with normal sound (98%). If not, it also gives you the choice to play the video with an external player like VLC which usually works well.
This work around has served me well but it still sucks! I wish the Ubuntu folks would talk to the Firefox folks and figure it out. This interrupts my work every day!