A:Answer According to Google, the Chromecast Ultra (even all the way back to the first generation) supports Dolby Digital 5.1. If it's another kind of 5.1 audio codec, then it most likely won't work. There are quite a few issues that people have complained about regarding this and it all boils down to hardware and what is supported to allow the Chromecast to pass the audio either directly to the TV to your receiver or just directly from your receiver. Your receiver has to support it as well. Here's a good thread describing exactly what you're asking:
https://productforums.google.com/forum/#!msg/chromecast/rIHlGdURY00/jkQgV_bOFAAJ
And if you're needing the technical specifics of the audio/video of Chromecast, here it is straight from Google:
https://developers.google.com/cast/docs/media
