Nightly build of 20150224 & Mixer & Burst Noise
I was trying out the different Sound Fonts I think they are called in the MuseScore Nightly Build of 20150224 with the Mixer & 'Burst Noise' and that caused MuseScore to be unable to play the score I had typed in.
However, restarting allows sound to play again.
Also, some sound fonts have what I can best descript as a bit of static. I'm not sure if this is in the sound fonts or has something to do with my hardware.