Android :: Audio Files Return 0 From MediaPlayer.getDuration()

Some audio files return 0 from MediaPlayer.getDuration(), even though they're perfectly valid files that play ok and that players on Windows can find the duration of. There seem to be two different cases here: certain file types such as .wma and .wav always return 0 duration, and others only do so sometimes. I haven't been able to find any pattern in the latter case, but it does seem consistent per file. The music player displays 0:00 duration for these, so it's not just my code.