Ras Pi 3 no sound through AVR but through the TV only?

Hi there long time Plex but new to Rasplex. So i have my PI plugged into my Denon HDMI port. My denon is plugged into my Panasonic TV. So for some reason the sound is only coming out of my TV speakers and not my AVR even though its plugged straight into my AVR. what settings do i need to change, i tried ticking unticking passthrough, there not a lot options either HDMI or analogue???