Generated narration time lag; no visible cause!

Hello!

I'm building a course, using the built-in text-to-speech tool. (Not the AI audio tool).

The slide has performed perfectly over dozens of previews, but today, during Preview, the lead narration (in the RED box) doesn't come in right away as it should, causing the second narration (GREEN box) bit to step on the first. Second narration and subsequent narrations come in when they should. When I preview the same slide by pressing the space bar, there is no such time lag on the first narration, but when I Publish this course, the time lag is there!

I removed, and re-added the first piece of narration: no change. I've also rebooted and run a gpupdate command suggested by IT when Storyline gets wonky.

Can anyone suggest other causes/fixes for this problem?

Thank you in advance..continuing to work the problem.