converting aac to mp3 via command line

The most appropriate command line tool is the ffmpeg utility available to be installed via the software center/synaptic manager.

A command line example would be something like this for Constant Bitrate Mode (CBR):

ffmpeg -i inputfile.m4a -c:a libmp3lame -ac 2 -b:a 190k outputfile.mp3

Or even better for Variable Bitrate Mode (VBR):

ffmpeg -i inputfile.m4a -c:a libmp3lame -ac 2 -q:a 2 outputfile.mp3

More information on mp3 encoding with FFmpeg can be seen here:

FFmpeg MP3 Encoding Guide

If your Linux complain that he doesn't have an mp3 codec, try this:

ffmpeg -i inputfile.m4a -acodec libmp3lame -ac 2 -ab 160k outputfile.mp3

Note that the -ab parameters takes in bits per second, not kilobits per second.