There is one incredibly valuable piece of info missing here, are you using a 1080p or 4k (that is HDMI 2.0 HDCP 2.2 compliant)?

If you're doing 1080p, there are lots of audio extractor solutions (cheap Tensun); however if you want 4k the ONLY solution that currently works is using a Key Digital HDMIFIX22 ($230-$250), and then converting from digital coax to digital optical ($50-$80)...only it imparts about a .5 second audio delay and there are no current solutions to delay the HDMI image.

- pricing is retail/MSRP

There is a Wyrestorm extender kit scheduled for release sometime Q1/Q2 and is an Extender with the capabilities and compliance built in...estimated price is $800-1100 (price was not officially set 10 days ago when I was researching a solution for the problem you have); the unit it replaces works...but wasn't HDMI 2.2 compliant and causes problems because of it.