Like nervousness, silence might not feel comfortable sometimes but it doesn’t necessarily mean something negative.
In your viva a moment of quiet could be while you or an examiner checks a detail or finds the right place in your thesis. You might need a quiet pause to think or read, or to make a note. Silence could be a side-effect of a video viva delay or a simple pause to settle after a noise from outside.
Silence in the viva is a brief quiet between questions, responses and discussion about really important things. Silence in the viva is not for long before the words flow again. Silence in the viva is just one of those things that will happen.
Prepare for talking at your viva by rehearsing and talking before your viva. Use that opportunity to prepare for the silence too.