After a few minutes the sound disappears. I have to close it and restart it. Then a few minutes later it does it again. It's a real pain. I really hope that they sort it out. It looks like it could be very good if it actually worked.
Really useful music theory drill. I would suggest a bit more tutorial as it takes a little while to figure out the exercises Also the triad test could just play the root note of the key, or at least play the scale a bit faster!