Simple solution would be to have a music track playing while you keep checking each one of the 6 rear audio sockets that you have available. One of them should give you full sound.
But before you do that you need to get into the Windows audio properties and check the Levels for the speakers and make sure they are all on full.
Sound Icon (Right Click)--Playback Devices--Speakers--Properties--Levels--Playback Control--
L,R,C,Sub,RL & RR should all be set to full.