How to Export an Audacity File As an MP3 in 2 Simple Ways
Audacity tutorial: Converting to mp3 or AAC and other formats, example: Matthew Smith - epic orchestral synth music for film and game effects; Buanzo's LAME mp3 library
Libraries Preferences - Audacity Manual
How To Add MP3 Support to Audacity (to Save in MP3 Format)
Change Audacity File Formats
How to Record Audio and Save as MP3 File using Audacity