There are no settings for Chromecast from the user’s side to setup 5.1 Audio.
Chromecast supports 5.1 normally.
Make sure AV Receiver Setting is as follows
i) Connect the Chromecast to the AV receiver
ii) Connect the video output from the AV receiver to the TV
Iii) Audio output should be in "DIRECT" and not in "AUTO SURROUND"