Speakers playing up,no sound coming out other than an irritating sound...?

i put in an after market stereo in my ford puma 1998 and it played ok for a week or two,then one day the speakers just started playing up.when I switch the stereo on,the speakers make a funny scratchy noise,and i cant hear a sound,but when i put it on fm,sound is there...what can i do to fix this??