Happend to me too, since a few month I no longer have sound on my video recordings for some reason.
(Pixel 7 Pro)
I also had this issue with the Voice Recorder app, but there I was able to change the mic source to back to "microphone" and it worked again.
I was also able to capture video with sound using a different camera app (Open Camera from f-droid), but there also I had to change the audio source too as it was defaulted to "Camcorder" which was not picking any sound.
So I guess that at some point, something happened that made the default microphone source being globally updated to something that was not supposed to be, and I was able to set it back everywhere but the default grapheneos camera, which does not provide an audio source setting.
I would love to have an audio source picker in the camera video settings, first it would allow people to choose the device they want to record with, but that would also greatly help people here to try to get their microphone be set back on the right source when they no longer have sound for some reason.