No Sound for TV apps thru Marantz AVR

and,it is probably your Receiver .Optical gets you going but,it would be nice to have the ARC working properly.I had to use Optical on My older Onkyo[TX-NR838] because it was hit or miss on ARC

Actually i found the setting on the Marantz AVR that allows me to play the You Tube music from the TV in either Dolby Surround or in Stereo. 

Thx for your help ....

Steve

YouTube is stereo only (expect some of their YouTube Premium/Red content).

To do pseudo-surround, switch the mode on the receiver from Stereo/Direct/etc. to Dolby Pro Logic II (or Dolby Surround if yours is newer).

It may be considered a sin here, but I listen to most all my music upmixed to surround (music mode, where vocals are duplicated to the center instead of extracted), and unlike All Channel Stereo, the surrounds only play the instrumental, and at a low volume so adds ambience without being distracting; I believe Dolby Surround doesn’t have separate modes, but you should still be able to change Center Width/Spread to adjust how it handles vocals, at least those that are meant to be centered).

First go to your Youtube settings and make sure is is broadcasting in multichannel.  Then do the same in the LG.

Bill

Thank you  - 
I adjusted my setting as suggested - and things are working better, but the youtube videos (played on the LG TV) are only being processed by my AVR in stereo through my front 2 speakers. 
I would like to hear the youtube videos through all 5 speakers.
Is there a way to accomplish this ?

Steve

HDMI ARC is one of the most unreliable technologies in existance. Optical should work though, make sure your Marantz is set to the right input and make sure the tv is outputting sound through optical by checking it’s audio settings.