I have set up the 5.1 speaker system and was all working correctly. Now only 2 of the speakers + the subwoofer seem to be working. There is no sound coming from the rest. I have swapped the speakers over and they work fine, so I know the speakers themselves are ok. All connections have been checked several times and seem ok also. I have checked every volume setting I can find, none of them are on mute. What else can I check? Thanks.