Power Audio Cutter 1.0 is a quite complete audio editor that lets you cut and convert among multiple formats as diverse as WAV, MP3, M4A, AMR, WMA, RM. MPA, APE, MPC, WV, AC3 or AU.
On each audio fragment you can divide it into the parts you want, joining parts even if they are of different formats. You can do mixes on all types of fragments and get a resulting file that joins them all together.
This makes it a practical tool for making all kinds of music mixes.