I did not write the (now generally unavailable) AllegroMP3 library - Javier Gonzalez did. However, the last version that he released surprisingly did not support the compressible, encryptable datafile API that the main Allegro library provides. So I modified it to do so. This version is available, amongst other places, here.
While working on Target; 2006, music playback became an issue. All of the game's data files were in a compressed archive format, apart from the music. Since I was not particularly fond of the messy-looking file layout this caused, I modified the open source ALMP3 library to fix this problem. The library is written in C, and released under the GNU LGPL.