There is probably a setting in the HD menu that turns on dolby digital(optical). If still no luck you can keep connected to the HD receiver and remove the optical cable from the yamaha receiver and look to see if there is a red light shining through the optical cable.

if still not getting sound, try the settings of your yamaha and make sure it is looking for the correct audio input(optical) and that it is looking for sound on the correct optical input.

hope this helps.