From this Bug report, it looks like libtunepimp (musicbrainz) has been built without mp3 support 
It is fairly easy to rebuild the package with this support. Most of this can be done via adept or synaptic.
This is for dapper, though it will be similar for Breezy, the libtunepimp package may be named differently
You will need to enable Universe in your sources.list for this
Step one:
Install libmad0 and libmad0-dev. this is where libtunepimp will get its mp3support when it is built
Step 2
Install apt-build, which simplifies rebuilding packages, and also will install all the tools you will need to do this.
There will be dialogs asking various questions, just use the defaults given by simply hitting 'enter' in every sectiom until you get to the CPU architecture selection, where you will need to choose your type. Mine is an Athlon XP so I chose that.
Step 3
Download , build and install libtunepimp
Open a terminal window (K Menu/System/Konsole) and enter the following command:
This will download the sources, build and install libtunepimp for you, and may take a few minutes or more, depending on your system.
If it gives an install error, you can find the newly created deb file libtunepimp2c2a_0.3.0-9.1ubuntu3_386.deb, which will be in /var/cache/apt-build/repository and you can install it manually. You won't need any of the other packages found there.
Step 4
Restart Amarok and test by trying to edit a track's information
It is fairly easy to rebuild the package with this support. Most of this can be done via adept or synaptic.
This is for dapper, though it will be similar for Breezy, the libtunepimp package may be named differently
You will need to enable Universe in your sources.list for this
Step one:
Install libmad0 and libmad0-dev. this is where libtunepimp will get its mp3support when it is built
Step 2
Install apt-build, which simplifies rebuilding packages, and also will install all the tools you will need to do this.
There will be dialogs asking various questions, just use the defaults given by simply hitting 'enter' in every sectiom until you get to the CPU architecture selection, where you will need to choose your type. Mine is an Athlon XP so I chose that.
Step 3
Download , build and install libtunepimp
Open a terminal window (K Menu/System/Konsole) and enter the following command:
Code:
sudo apt-build --reinstall --force-yes install libtunepimp2c2a
If it gives an install error, you can find the newly created deb file libtunepimp2c2a_0.3.0-9.1ubuntu3_386.deb, which will be in /var/cache/apt-build/repository and you can install it manually. You won't need any of the other packages found there.
Step 4
Restart Amarok and test by trying to edit a track's information