Even though I have checked the boxes, in general settings audio page, for digital in mythtv's audio setup, my receiver only indicates it is getting a prologic signal via the spdif cable. If I set mythtv to also upconvert stereo to 5.1, then my receiver indicates Dolby Digital for everything I play, so I know it can work.

I have mythtv's audio device set, in general settings audio page, to my nvidia motherboard's spdif output, and in the advanced section, it is set to default. I read a suggestion to turn off mixing (on the next config page) but that didn't help. I have left it off for now.