Comment 149 for bug 111257

Here is a summary of the workaround mentioned before the bug was fixed:

1) Open a terminal window (Applications > Accessories > Terminal)
2) Enable the Multimedia Systems Selector menu option:
a) Go to System > Preferences > Main Menu
b) Under "Menus:" scroll down to and click "Preferences". The right part under "Items;" will update itself
c) Mark the "Multimedia System Selector" checkbox, click "Close"
3) Go to System > Preferences >Multimedia System Selector"
4) Under the "Video" tab change the Default output plugin to "X Window System (no XV)"
5) Click "Test", you should see a window popup with a series of vertical color bars, much as older classic TV color tests
6) Click "OK" to finish the test, now mpeg playback should work.