The feature you're looking for here is the 'lock soundtrack' option, which has been deprecated. While that feature did have some great uses, it also resulted in more confusion than expected. I believe the intention is to restore the feature once it's sufficiently improved.
I really appreciated this feature because it always syncs the soundtrack with the slides even when I change the slidtime. Suggestion for the future: Don't synchronize the soundtrack with the slide, but with the exact time in the timeline.
Best regards Dieter