RingtoneEditor si a software that lets you edit MP3 ringtones to your exact liking.

You can import your newly created MP3 ringtones or select a CD track and select the exact location and length of the final ringtone before uploading to your phone.

You can select and play as you edit so that you get the exact ringtone you want to have.

You can even change the bit rate of the selection to have better file size control.