Fossperson I'm not sure if you are still having this issue, but I was able to resolve this on my phone. I followed what ErnestThornhill's link mentioned, going to my phones Settings > Security > Exploit protection.
This still didn't work for the camera that came with GrapheneOS, so I downloaded Open Camera from Fdroid: https://f-droid.org/packages/net.sourceforge.opencamera/
In the camera, open the settings by opening that app and clicking the settings icon in the top right. From there, click on "Video settings..." Scroll down and click "Audio source" and switch it from "Camcorder" to "External mic (if present)".
This worked for me. Hopefully it helps anyone else who has a similar issue.
For the record, I have a Pixel 6 with Android 16.