either the speakers are just dead...or (+) is crossing with (-) internally in the speakers or frayed wire. best thing start off by unpluging one of the speakers..then turn on the radio and see if you still hear the crackling...if you still hear it put that one back in and unplug the next one.. repeat till you get all your speakers. this will determine if the speakers are dead...if you happen to unplug one of the speakers and the everything goes back to normal then thats the bad speaker.
based on the crackling and sometimes no sound...i would assume the speakers are just dead..