Launch the terminal, access the folder containing your WAV file, then issue this command:
lame file.wav file.mp3
Replace file.wav with your own WAV file, and replace also file.mp3 with the name of the output MP3 file you want to generate.
The MP3 file will be generated in the current folder.
Note: This command is tested under Ubuntu 10.10. You can also use this command to convert to other audio formats (mp4, ogg, midi, etc.)