I found myself needing to convert a wav file into mp3 format recently. Using Linux I was easily able to accomplish this using ffmpeg. FFMEG is a complete, cross-platform solution to record, convert and stream audio and video. It includes a command line interface for just this sort of need.

Converting a wav to mp3
I was able to install ffmpeg using Ubuntu apt-get:

sudo apt-get install ffmpeg

Once installed I was able to convert my file recording.wav into mp3 format using the following command:

# ffmpeg -i [input_file] [output_file]
ffmpeg -i recording.wav converted.mp3

You should see some verbose output indicating if the conversion was successful or not. Happy converting!