Lots of overruns for basic audio playback on Beagleboard

I'm struggling to run a very simple model involving audio playback on a Beagleboard xM without getting frequent overruns. Here is a link to a simplified version of the model (available until 1st of May):

https://fluff.bris.ac.uk/fluff/u3/mexdw/imM_JIZh4aBFQnMk4FVlXAFFS/

All it should do is send a sine wave to both audio channels. Even with the audio frequency set to the lowest option, 4000 Hz, I get a steady stream of overruns. The audio does play, albeit with some crackling from the overruns, so the basic functionality is there. I would have thought this would be well within the Beagleboard's processing ability. Can anyone tell me what's going wrong?

Thanks in advance for any help.

David

P.S. I'm running Matlab R2012a in Windows 7 (64bit)