My devices and equipment:
Vizio D65u-D2 65" 4K UHD Smart HDTV
Google Chromecast Ultra
Samsung Galaxy Note 5
Running on Comcast Xfinity Extreme 150 (but only getting max 29 mbps on the Vizio and can't figure it out)
Network = 5GHz isolated
Router = Apple AirPort Extreme
Modem = Arris sb6183
My issue occurred when trying to cast the Google Play movie trailers in the Google Play App from my Note 5 to the Vizio TV.
There was video but no audio.
I could 'reboot' the Chromecast on the Vizio (via the Google Home App on my Note 5 in the 'card' for the Vizio, under Devices, click the 3-dot icon top right for menu/Settings options and select 'reboot') but that would only correct the audio for 1 Play of that trailer. If I tried to play any other content, the audio was once again missing.
I read these posts and played with Vizio Settings.
Vizio Menu >> Audio >> Analog Audio Out >> select "Fixed".
That should not have any effect on the Chromecast Ultra or casting, but it does.
I hope this helps others - Cheers!