I had exactly the same problem when I first set up my Panasonic DMP-BDT110 (I'm assuming that the 310 will be the same in terms of set up).
A couple of points first: I have my blu-ray player set to audio 'bitstream' out, so that the AVR does the audio decoding. Not all blu-ray discs default to HD sound, so some (not many) you have to go to 'Top Menu' and change the audio preference to HD sound.
Most important, though, is a setting on the DMP-BDT110 (310) that is set as default (crazy I know) that forces HD sound to be output as standard DD or DTS 5.1. The setting you need to change in the Setup Menu and Player Settings is:
'Sound' menu and 'Digital Audio Output' you must set 'BD-Video Secondary Audio' to 'Off'.
Also be aware that my Onkyo 609 has a 'Listening Mode Preset' and I have all the DD & DTS (including HD sound) lines for the BD/DVD set to 'Straight Decode'. Same for the CBL/SAT for my Syk+HD box. If you do change the 'Listening Mode Preset' you need to power cycle the Onkyo for it to take effect (from the remote will suffice).
Hope this helps.
Paul.