well... iTunes and VLC. At least for The Slip anyways. It won't work in WinAMP or realplayer. I also have some AAC files that won't play in Musicbee, but play fine everywhere else (iTunes, WinAMP, VLC, realplayer). It might be a possibility that the .m4a files are in fact truly FLAC files, and I'd be quite interested to see this version of MusicBee would work this problem. If not, then convert will have to do!
EDIT: Well, I had two Kelis songs in AAC that wouldn't start in MB, but everywhere else. So I played it, but both would cut off well before the song was over, even if I advanced to the end. So for those two, at least, it's the file, not MB.