Android - How to convert pcm/wav to mp3?
The LAME encoder is useful for converting such files. PC applications can be found here: http://lame.sourceforge.net/links.html
You can try another recorder, like Hi-Q MP3 Recorder. This will record it immediately in mp3 format.
you can use an online converter to do the job. Try for example http://www.mediaconverter.org