Problem: When playing back flash video in firefox on Ubuntu version 12.04, you may experience stuttering audio and dropped frames
Possible cause: pulseaudio may be configured for “simultaneous output”, which adds overhead that may be slowing down your system.
Solution (in my case): Click the volume icon, then click Audio Settings. Make sure the default output device is the correct sound card, not “Simultaneous output.” Also make sure the settings for the sound card are correct, including inputs and codecs. Don’t turn on anything you don’t need.