We just bought a Philips 5.1 Home Theater System. The problem we're having is the 5.1 speakers only work with dvds. We can only get 3.1 to work with regular cable. We have 2 sets of the 3 color cables (red, yellow, white)- one hooked from reciever to tv for the tv audio and one set from the reciever to the tv for dvd audio. Why won't 5.1 work with regular cable? Thanks!