In the features page it says only .wav and .ogg formats are supported, but not .mp3! Please include .mp3 format support in the Esenthel engine. Thanks!

I'm not sure about this but MP3 is most probably a patented technology which can't be used for free.
OGG format is much better for games (has better quality with smaller files - I use 96k bitrate)

Know any free audio programs that allow you to convert a .mp3 to a .ogg or .wav?

I use GoldWave tool <!-- m --><a class="postlink" href="http://www.goldwave.com/">http://www.goldwave.com/</a><!-- m -->