Hey @Geo80! Does this issue happen on normal calls as well? Make sure that the phone is fully up to date by heading to: Settings > Software update > Download updates manually. Next, head to: Settings > Apps > The apps you are having this issue with > Storage > Clear cache. Is the issue still there?
If it happens only during video calls online, this may be due to poor internet connection or the signal not strong enough. Also, if it happens only with WhatsApp then I would suggest getting in touch with the app developers or WhatsApp support.
I see, please remove any headphones, earphones or other audio devices inserted into or paired with the phone. After this, test the receiver: from the phone dialler, enter *#0283# > Select RCV Packet Loopback ON > Say a few words as if you were on a call. Can you hear your own voice?