I recently started having an issue with YouTube playing a video without sound for 2 seconds, then stopping. This is a new laptop, so everything is fresh and new, leaving me with a ton of potential culprits: OS X, Safari 3.1.1, Flash, network, Perian, etc….
Googling showed that other people were having this issue across many OSes and browsers. Unfortunately, most of the advice was “reboot” or “change your network buffer” or “YouTube sucks!”. None of which helped me.
What did help, is downgrading from Flash Player 126.96.36.199 to 188.8.131.52 using the Adobe archive of Flash Players. I uninstalled the 184.108.40.206, and installed 220.127.116.11, and poof, no more problems with YouTube videos.
I hope this helps someone.