I used a utililty called MergeMP3 along with a sound file with 2 seconds of silence (you can record your own) plus a utility that converted .wav's to .mp3's.
If you are using all mp3's, you don't need to convert from .wav to .mp3, of course.
The reason I went to all the trouble of merging sound files is that a common complaint with Vibrate and Ring programs is that the sound file plays only once. So my short ringer would only ring once (for 2 seconds). So I wanted to stitch together something longer. So basically I copied the short ringer and renamed the copies, e.g. ringer1.mp3, ringer2.mp3, ringer3.mp3, etc. Again, although the files are named differently, they are really the same file.
Then I used MP3Merge and joined together the files into a single file:
ringer1.mp3 + silence1.mp3 + ringer2.mp3 + silence2.mp3 + ringer3.mp3 + silence3.mp3, and so on, to make it about 20 seconds long.