You are in for a world of trouble in trying to convert to mp3's. There are expensive patent licenses involved. You can find ton's of questions of people looking for a solution to this, and no good responses.
If you are looking to just do something for yourself on your own, you can try FFMpeg, but they have some pretty serious legals disclaimers here:
This line is particularly worrisome:
Q: Is it perfectly alright to incorporate the whole FFmpeg core into my own commercial product?
A: You might have a problem here. There have been cases where companies have used FFmpeg in their products. These companies found out that once you start trying to make money from patented technologies, the owners of the patents will come after their licensing fees. Notably, MPEG LA is vigilant and diligent about collecting for MPEG-related technologies.