I’m trying out Ardour and like it very much, except that I can’t find this option, which is a bit of a breaker since I send work via email to clients around the world and I have confidence that mp3s will be easily sent and played.
I’m working around by exporting as .wav and converting in Audacity but it’s not an ideal workflow.
Mp3 is a restricted codec, so I doubt it will be integrated into Ardour. You can export to Ogg, though.
And when Mp3 is necessary, I personnaly use Soundconverter ( http://soundconverter.org/ ) which is in (at least) Ubuntu’s repositories, and is very handy and fast for such a task : Right click on your exported wave > Open in soundconverter and click “convert”. (You’ll have to set your preferences to mp3, chose the bitrate, target folder etc… the first time you use it).