If you wish to use the bug to set a ring tone, then it must show up in the music player. For a file to show up in the music player is seems that it must be an MP3 file, and over 500kb in size (or 30 seconds).
The ring tone I wish to use is a 3 second amr file, so this is what I did.
Convert the amr file to a wave file for editing
ffmpeg -i josie-ring.amr josie-ring.wav
open the resulting wave file in an audio editor and duplicate the audio until it loops past 30 seconds, then save over the wav file.
Convert the wave file to a full size music mp3
ffmpeg -i josie-ring.wav -ab 128 -ac 2 -ar 44100 josie-ring.mp3
Finally send the resulting mp3 file to the phone (I use bluetooth, but I could also put it on the memory card and transfer it off that onto the phone).