To get around this, you need to install
gstreamer0.10-plugins-ugly-multiverse, which fortunately is in the multiverse repository.
To install the plugins, run the following command:
sudo apt-get install gstreamer0.10-plugins-ugly-multiverse
It's a short download, only around 40kB worth of files.
From the Gstreamer web site:
GStreamer Ugly Plug-ins is a set of plug-ins that have good quality and correct functionality, but distributing them might pose problems. The license on either the plug-ins or the supporting libraries might not be how we'd like. The code might be widely known to present patent problems.