Convert the mp3 too a wav first.
[youri@nemesis ringtone]$ mplayer -ao pcm Loudest_Ringtone.mp3 -ao pcm:file="ringtone.wav"
Then wav to m4r. M4r is the format iphone wants ringtones in.
[youri@nemesis ringtone]$ faac ringtone.wav -o loudest.m4r -w
Transfer file to iphone. I use sftp.
[youri@nemesis ringtone]$ sftp root@yphone:/Library/Ringtones
root@yphone's password:
Connected to yphone.
Changing to: /Library/Ringtones
sftp> put loudest.m4r
Uploading loudestm4r to /private/var/stash/Ringtones.wljzz0/loudest.m4r
loudest.ringtone.m4r 100% 306KB 305.8KB/s 00:01
sftp> exit