I have been trying to get 5.1 audio from Chromecast for a week now since it showed up at my door. Until tonight when I noticed that I wasn't getting DTS or DD from my Sony Google TV either, I had dealt with this issue before so I knew exactly what to look at and sure enough under the HDMI menu on my Sony STR-DH800 Audio out was set to AMP+TV, in this mode it will only do PCM audio, something to do with HDCP. Anyway I switched the audio out back to AMP only, and I tried a 5.1 Title from Netflix. Low and behold the screen displays Dolby D + instead of PCM this time.
Just replying to say thank you for this and in case anyone else is googling it - it fixed the problem with only getting stereo from Chromecast on my Sony STR-DH820. Got DD+ straight away after changing to amp only.