First check the 2910's digital audio out is set to 'normal', then make sure the 3805's input is set to 'auto' and not 'PCM', 'DTS' or 'EXT IN' (this is at the bottom of the remote.
Also make sure the 2910 is outputting audio over optical when you are using the HDMI at the same time(as audio can pass through the HDMI also) check the manual if there is a setting for this or if it passes over both by default.
Also check the Zone settings in the 3805, it may be sending the dvd input to another zone.
Has that optical cable worked before, it's possible it may be defective, try using the coax digital connection.
cheers