1) be sure to go into audio set-up on the dvd's menu and select the 5.1 soundtrack
2) try disconnecting your DVD analog out cables and just use digital until you can lock in on the 5.1
3) try to find a way to display the output going from your DVD player (for example, on my player I can choose bitstream output, two-channel PCM, or analog out -- you should select bitstream)
4) go into your receiver's set-up and designate the type of digital input you are using (toslink or coax). Once you select the type of digital input, you may need to specify which of the multiple toslink or coax inputs you are using, as many receivers have multiple coax and toslink inputs
5) when playing DVD's, select "digital" input rather than "analog" on your DVD player
This should solve things -- I can't think of any other potential problems other than a defect w/ your gear.