If you can't play MP3 files on Ubuntu 11.10 using Banshee, then follow these instructions that may fix your problem:
1. Launch the Terminal and run these commands:
sudo apt-get build-dep banshee
sudo apt-get install gstreamer0.10-plugins-good gstreamer0.10-plugins-bad gstreamer0.10-plugins-ugly
2. Restart Banshee and you will be able to run MP3 files. Enjoy!