Audio sticks when stop button pressed

I’m trying to work with an imported wav file of a piano backing and adding a vocal line for one of my choir rehearsals. Each time I press the stop button in the transporter the audio locks and repeats the last sample over and over again.

Using Windows 7 SP1, Cubase AI5, MAudio Audiophile 2496 card

Advice nedded urgently

Thanks

Bumpmusic