There's a GStreamer bug (in the gstreamer0.10-plugins-bad package) in the latest Ubuntu 12.04 Precise Pangolin and other possibly distros that breaks MPEG4 (.mp4) playback for applications such as Totem, Minitube and others.
Luckily, there's a work-around for getting MP4 playback working (tested under Ubuntu 12.04) - renaming the "libgstvideoparsersbad.so" file:
sudo mv /usr/lib/i386-linux-gnu/gstreamer-0.10/libgstvideoparsersbad.so /usr/lib/i386-linux-gnu/gstreamer-0.10/libgstvideoparsersbad.so.bak
sudo mv /usr/lib/x86_64-linux-gnu/gstreamer-0.10/libgstvideoparsersbad.so /usr/lib/x86_64-linux-gnu/gstreamer-0.10/libgstvideoparsersbad.so.bak
That's it, MP4 videos should now work under Ubuntu 12.04. There aren't any visible side effects to using this work-around, but in case you find one, you can simply rename "libgstvideoparsersbad.so.bak" back to "libgstvideoparsersbad.so".
Update: this bug still exists in Ubuntu 12.10 and the above workaround works for Quantal Quetzal.
And speaking of Minitube, there's a new version out, available in the main WebUpd8 PPA: Minitube 1.8.0 Released With Quicklists Support, Compact Mode.
via Minitube forums