Fix issue #603, playback rate couldn't be changed on IE9

This was due to the test file (`silence.mp3`) having a very low sampling rate, which IE9 apparently couldn't change playback rate for. The new `silence.mp3` file is 44.100Hz. Futhermore I've added listener removing when the tests finishes

This should probably also be pulled into `master`, but it seems there might be some merge conflicts. I'll take a look