Hi there, i am having the same problem here! I am using it in the same way as you! Have a Bravia W4500 (52") tv. I am conecting the WDTV box via HDMI and the sound via S/PDiF directly trough the amplifier and i have no sound in the case of .mkv files wich have DTS sound! My amplifier is a DTS sound capable decoding 6.1 Chanel Sony amplifier (STR-DB790).
The problem is not with your amplifier! Hopefully the WD team will come up soon with a firmware update wich solve this bug… until then i am using a sound converter which converts (you can keep DTS sound on a separate chanel on file as well) DTS to Dolby Digital. Dolby Digital sound format the WDTV box 1st generation is capable to play it.
So you can watch full HD .mkv files on your WDTV box, unless DTS sound… but hey! the image remains same good and in this way you will be able to get sound as well!
program link :
regards from Athens - Greece! good luck!