Nest is asking me to send them before and after feedback snippets for
troubleshooting. I made them aware others do not have the audio option.
See if that helps resolve it for others like yourself.
You were on the right track from the get go. My cameras do not support
audio. No mic in any of them. I have two systems. Both don't have mics.
Both have this option. Actually you can enable it in the app as well.
Check here