Audacity You will need this AND Lame (below) to complete mp3 assignments. Both are free downloads.
Audacity http://audacity.sourceforge.net/
Lame You can find the instructions for installing Lame into Audacity in the Handouts link. http://www.free-codecs.com/download/Lame_Encoder.htm
Royalty Free Music http://incompetech.com/m/c/royalty-free/