Comment 11 for bug 226801

I had the same issue with Karmic. When I first played an MP3 the plugin assistant suggested 3 different packages to install. I only chose to install one of them (the ffmpeg one), and as a result Totem could play MP3s but cannot seek in them.

Having opened 'Ubuntu Software Centre' (the new name for Add/Remove Software), I searched for Gstreamer, and installed the 'Gstreamer Extra Plugins' package. Having installed this, and restarted Totem, the MP3 seeks and plays correctly.

I hope this helps someone else :)