At the end of the slider at the bottom there is a tiny fast-forward button labelled 'Fast Play'. When pressed, the music or video will play faster. If this button looks like it is depressed, click on it until it is no-longer pressed. This may take several clicks, as there are several speeds you can set it to.

Also, on the drop down menus at the top, go to Play>Play Speed and make sure it is set to 'Normal'.

Some of these settings have keyboard shortcuts, which you could perhaps have accidentally activated.

I hope this solves your problem!
Christopher J. Wright