No Sound for TV apps thru Marantz AVR

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.

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

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

Bill

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).