When trying out the starfighter tutorial I get "File contains data in an unknown format." when trying to play the m4a formatted file on a Windows machine. The file plays fine in media player. Is m4a no longer supported?
FMOD could play back lots of file types but had a very complicated license -- it was not even possible to license the FMOD3 API anymore. Audiere was free but crashy. Now Gosu can basically only handle WAV and OGG on Windows and Linux. :(
OpenAL does not parse audio formats, only play them from memory. Gosu uses libsndfile (& libogg) to translate files to audio buffers, but it can't handle M4A.