Hmmm…all seemed well with my Ubuntu installation until I tried playing an AVI file the other day. Of course I was in a hurry, and needed to view the file quickly, so Totem decided to play 2 seconds of the video file and then stop. I could put the seek pointer to anywhere in the file, and the same thing happened!
Very frustrating – especially since I’d tested video playback after installing Ubuntu and everything seemed to work! But, being the patient and kindly soul that I am, I managed to contain my rage to just shouting at the screen. No humans, animals, or (valuable) inanimate objects were harmed.
The fact that video playback had worked and then stopped working confused me a bit. But the answer was relatively simple. After installing Ubuntu, I then went on to install all the whizzy compositing effects – wobbly windows, 3D cube to switch desktops etc. And this, it seems, was causing the problem. One solution, then, is to disable compwiz; however, as any fule kno, once you’ve experienced the joy of wobbly windows, you can’t go back! So a better solution is as follows:
Open a terminal window and type:
gstreamer-properties
Click on the Video tab and change the Default Output plugin from Auto Detect to X Window System (No Xv)

Close it, and test video playback again. If it still doesn’t work then restart X Windows.
After this, video playback works flawlessly again. So I’m having to find other things to shout at now.