This is nearly always a symptom of one of the speaker connections short circuiting so these should be checked out in case a cable clip etc has pierced the cable, however if this doesnt cure the problem then there isn't really anything you can do as the unit requires testing to determine the reason for the problem, and which could be caused by a defective audio output IC etc etc. All this said on the basis that you are using the speakers supplied and have not added anything, as this would cause that symptom.