Backing Tracks Distorting

[NeilFann]

Hi, I've recorded backing tracks we want to use. They are wav files. When I play then, I can always hear them fine but half the time they sound terribly distorted to my singer. There is no discernable pattern to what causes this. The pattern is reversed when she plays the track, it's fine for her and messed up for me.

Any ideas, this is such a pity, we were looking forward to performing with them.