Thanks for testing this, Nat. Yes, I have checked (and double-checked, and checked again..!) the clip speed, which is 100%; I've never had any reason to change this. This problem has only occurred since installing the most recent version. I use this editor only very rarely; the previous video I did was 2013. I didn't need to upgrade, I just thought I may as well before starting my current modest work. I used my previous video as a base, or template, for my current project. I'll start again completely from zero and see what happens.
This is only for a few minutes of soundtrack over a photo for some colleagues on another forum, so is of very low importance in the grand scheme of things. I've not yet really started, so nothing has yet been lost nor wasted (well, a bit of time on your part and a couple of evenings for me, but, being retired now, my time hardly counts...).
Leave it for now with a question mark, and I'll try again from a clean slate. I gather that this is not a frequently recurring, well-known 'feature' (read 'bug'..!); that, at least, is comforting, I suppose.
I'll be back in a while with the result of my trials.
Thanks again; back soon.
Douglas