Music Composers Unite!
Caj, you should first make a WAV (or AIFF) file and then convert it into mp3 format. The mp3 format is a result of compressing (concernuing memory usage, not the sound dynamics) of standard audio formats. So, find a way to produce a conventional audio from your DAW, and then compress it to mp3.
to save your work and sounds to mp3 you need to dl the codec from reaper just try and save something to mp3 in reaper and it will send you strait to web site for dl