You have to select the multi channel audio in on the receiver. Use the source button on your remote or the front of the receiver to cycle to the correct source as per the manual info below:

6-Channel/8-Channel Direct Input
• There are two input choices available for use with
sources such as a DVD-Audio or SACD player
that are connected to the 8-Channel Direct
Inputs. Select the appropriate input for your
system and source equipment:

2xM80 VP180 2xQS8 2xM3 HSU STF3 LG 60PS11 Denon 3808 ATI 1506 LCR 2xATI 1502 Oppo BDP-83