I was reading through the manual page about Manipulating Sounds. I came across the first of the three issues with looping mp3s a while back with another project, and would like to suggest updating the following information:
Some MP3 encoders contain a bug where they add blank space at the end of the sound. This causes a skip during looping. Try using a wav instead.
Due to the nature of the MP3 format, files encoded either at a low bitrate or by a bad encoder may have a small amount of blank space at the start and/or end of the file, creating a skip when looping. Try using higher quality mp3s, look up a utility called mp3loop which is designed to remove this padding, or switch to a different audio format.