Welcome to Android Central! So you're using Windows Media Player to try to sync m4a (I'm presuming these are iTunes tracks) to your phone? Are those iTunes tracks DRM protected? If so, they can't be played on your phone. But besides that, I don't think WMP works very well to move music to a phone. Have you tried removing your microSD card, inserting it into your computer (using a fullsize SD adapter, very cheap to buy if you don't have on), and simply dragging and dropping the music files using Windows Explorer?