I would recommend Sound Converter. It is extremely simple to use for what you want. You can find it in the Software Center, or install it from the terminal:

sudo apt-get install soundconverter

Ubuntu Repo: soundconverter

Apt Link: Install soundconverter

All you need to do is open it up, change your preferences (Edit -> Preferences) then click the "Add File" or "Add Folder" buttons to add the music to be converted, and click Convert. It doesn't get much simpler.