heres the deal...
download audacity - http://audacity.sourceforge.net
in Preferences, goto Quality and change the default sample rate to 22050, and in File Formats change the MP3 bitrate to 32
open an mp3 with audacity...
what i do is:
* select the section you want the ringer to be
* copy it and open a new project in audacity
* paste your selection
* place the cursor at the very beginning of the file and Generate|silence
* trim the silence to 1.25 seconds, so there is some silence at the beginning of the track
* split the stereo track
* make both tracks mono
* export to mp3