I just posted about this issue at the “Idea” forum, but I thought I should repeat a summary here in case anyone else has the same problem.
One reason why I chose the WD TV HD player over competitors is support for dvr-ms files. For those not familiar with these, that is the container format used for HDTV recordings done with Windows Media Center. When the broadcast has DD5.1 audio, the recording plays back (through Media Center or a compatible software player) the surround audio.
I have my WD TV player connected to my receiver with a toslink cable, and the unit passes through digital surround audio from MKV files and other containers with embedded digital audio. However, when I play back a dvr-ms container (with confirmed DD5.1 audio), WD TV apparently downconverts the audio to stereo rather than pass through the digital surround audio as it does with other containers.
I do like this player very much and I am looking forward to future firmware updates. I hope they will fix the digital audio passthrough for dvr-ms files.