If the source of sound fails, meaning the device doesn't send sound:

When sound recording is active (mike or desktop) it will generate a tiny file with nothing inside.
When sound recording is OFF , it will record the video as normal. Checked with full screen video and select screen area options. With mike or desktop or bone or none.
I have this issue since some time ago, I thought it disappeared but now that I activated the sound , it has come back.