Unwanted click noise on playback of muted movies in Keynote

I've been a Keynote user since the very beginning and something of an old pro. But recently, I've been getting a minor problem which I can't solve. I often use movies with the sound muted completely, so that I can talk over them. On some movies, but not all, there is an annoying click or 'tick' sound as the movie starts. My movies are all either .mp4 or .mov files and most are quite old and moved from deck to deck as required, which means it's hard to find the originals! The noise happens on both file formats but not on all files. Where I have the original, I can run it in Quicktime and there is no noise. Re-import it into Keynote, turn down the volume, and the click is there, right at the beginning. So, it seems to be a Keynote thing. And maybe there are different flavours of .MP4 and .Mov which is causing this to happen on some files and not on others?

I have a new MacBookPro to replace the older one (which had a faulty Touch Bar, but is otherwise identical) and am running 11.5.2 and Keynote 11.1