sounds to me like it's the wrong cable, it's probably a mono cable or not making a full connection... that or your balance is off in the stereo

Check the cable ends, do they have three contacts [good] or two [bad] (separated by the plastic bands on the tip)?
Is your balance set wrong?

I'd say if your balance is right then just get another cable to try, they're cheap at radioshack... make sure it's stereo