What player are you using? m4a is roughly mp4 or mpg4 audio as a very rough & slightly inaccurate description of them. It's a newer format. Players that can play m4a are 1. iTunes, 2. Quintessential (QCD) after downlding extra plug-in, 3. WinAmp, 5. WMP after downlding the 3ivx codec. And there's plenty more, I'm not sure of the complete list.
See http://www.m4a.com/ for a description of the format. Another place to look is http://en.wikipedia.org/wiki/M4a