No MP3 playback or AVI playback

[Dave M G]

Ubuntu Users,

I'm Using Dapper Drake.

I dutifully followed the instructions on this web site:
https://wiki.ubuntu.com/RestrictedFormats#head-a57167a3ce442dc52d9b05e46a14503330d4e970

... and yet still, Amarok will not play back MP3s, and Xine will not 
play back almost any AVI files.

With Amarok, it just glides through the list of songs without playing 
any, or returning any errors.

With Xine, it complains that it can not play anything encoded with an 
Xvid codec.

Is there a bug with Dapper, or am I still missing some crucial component?

--
Dave M G