App was working nicely until it started playing a music soundtrack, completely out of the blue. How am I supposed to concentrate on the listening audio now? I've found no way to turn the music off which is completely inexcusable, so most of the time I'm forced to turn my phone's sound off. This obviously cripples the learning. Developers: give us volume control over the music and the ability to mute it ASAP. This is just bad design.