In DD mode, stb will transmit the sound through either coaxial out or optical out. Thatswhy you are not getting sound.
To exp DD , u need 5.1 system or home theater with coaxial or optical input.
Using converter and normal audio mode doesnot make much difference.
So jus adding coaxial or optical cable can make things work for me...as I require Dolby Digital audio in D2H..