As long as you have a WAV or MP3 file, the SoX package, and the GStreamer gstreamer-plugins-ugly package installed, you can use these scripts to convert from MP3 or WAV to AMR, and back from AMR to WAV. Perfect for creating ringtones for your mobile phone out of sound clips or favorite songs!
http://rudd-o.com/archives/2006/09/18/convert-wav-mp3-files-to-amr-in-linux-using-gstreamer/