Please see the explanation concerning passthrough of audio from an HDMI source via a TV to an AVR: Can The Vero 4K Support The Updated HDR10+ Dynamic Metadata Spec? - #15 by Chillbo
The only suggestions here would be: You’d need an HDMI splitter that sends the same signal to the TV as well as to the AVR. Then you can playback the passthrough audio via the AVR and have the 4k video passed to your TV.
The only problem here would be that the splitter wouldn’t be allowed to send the 4k video signal to your AVR via HDMI, but only to your TV - otherwise the AVR would refuse to take the signal at all… Or even make the Vero downgrade video to 1080p like it would with a single passthrough HDMI connection through the AVR to the TV.
Maybe somebody else who’s using a setup like this could chime in here and recommend a splitter which can downgrade the video on one of the two outputs or just send audio on one of the two HDMI ports (some HDFury device maybe?). What has to be achieved here to make this work: something like a dual HDMI out like some higher-end UHD Blu-ray players offer for 4k video and separate audio via HDMI.
I’m not sure what kind of splitters are out there, but I have the feeling that you might need to upgrade your AVR eventually to get 4k video and HD audio at the same time.