MP3 to CDA
This is very simple.
Nero (and most other CDR software) will do it for you.
Create new disk as an Audio CD.
Drag the MP3 files (or .wma, etc etc), then burn the disk.
My entire CD collection was ripped to a removeable harddrive as MP3's so I can listen to them while working away. I then burned then to CDs to listen to them in the car.