In order to convert amr audio files to mp3 (or wav/ogg), one way is to use ffmpeg in combination with the lame encoder.
The easy way is to use synaptic, enable multiverse repositories, then install ffmpeg package and the ubuntu-restricted-extras package which will install a working lame encoder along with other “goodies” like Microsoft core fonts, unrar and various useful audio codecs and related apps. Be aware that installing lame with:
sudo apt-get install lame
didn’t work, the ffmpeg complaining about a non-existent mp3 encoder (lame).
I have converted an amr to mp3 file using (in terminal):
ffmpeg -i input_file.amr output_file.mp3