Those .m4a files don't play very well with mplayer or mpg321 however a solution for debian users exists. The dir2ogg package can convert this .m4a format and other formats into .ogg files. For .m4a the file first gets converted to .wav format and from .wav to .ogg format. The command line podcast uses .m4a files and surprisingly is worth a listen if you're at all technically inclined.
