BDP-S350 AVCHD audio problem

[pull10k]

I have a Sony BDP-S350 and an Onkyo 905 receiver. I use DSP pass through for my Blu rays and it outputs bitstream perfectly.
But when I use it to play my AVCHD discs it only its internal codecs and outputs Dolby Digital. Obviously I would want my $1600+ receiver to decode instead. I cannot figure out why it does this.
Does anyone else know of an answer? I'm stumped.