Unfortunately amaroK doesn't come with MP3 support in Ubuntu. But that's easy to fix.
1. Check, if you have included the Universe sources in /etc/apt/sources.list
2. Type "sudo apt update" in the shell (or use the Update Button in Synaptic)
3. Type "sudo apt install gstreamer0.8-mad" (or select and install it in Synaptic)
This will install the needed libmad0 library as well.
4. Start amaroK and play your MP3 files :)