Blender is not only a 3d Modelling Software, but also a great OpenSource Video Editing Software. It's also decent for quick Audio Editing. I used it for an Audio Drama project, since I didn't want to use expensive Software.
Since it isn't very obvious on how to export your project to mp3, here is a little tutorial:
1. Go into the Render menu in the Properties tab
2. Select MPEG as fileformat
3. In the Encoding options, select Mp3 as Format
4. Additionally you can change the Bitrate
P.S.: I spent hours of searching and trying before I found this option. You could also use this to extract Audio from Videos and arrange them together (severall Youtube Video Parts to one mp3).
