Mac: No audio in Flash? Trash your preferences!
I was trying to play a YouTube video on a Mac and wasn't getting any sound. After Googling around, I found that I wasn't the only one running into this problem and that the solution is to trash your Flash Player preferences under the ~/Library/Preferences/Macromedia folder.
[Update] After trashing preferences, the video was still stuttering. Apparently the problem was with the Flip4Mac WMV Internet plug-in. The solution was to disable it from System Preferences.