FFMPEG is not trimming the audio to the length of the video
In the process of submitting a bug report, I tested the same input samples on the nightly build of FFMPEG of 11 February 2020, which fixed the problem. That nightly build had version:


ffmpeg version git-2020-02-11-f15007a


To install a nightly build, I suggest using the compiled binaries from Zeranoe. Beginners should choose "Static" libraries, which includes the required libraries in the build.

I suspect that the problem is due to the input samples being slices of an hour-long footage, which is longer than the EU restriction on camcorders. Using the nightly build avoids this case of sound and video out of sync even with shortest clip trimming.

For the EU restriction, see Filesystem that records longer than 4 GiB, or one hour without interruption or Why is there a limit restriction to the 1080p film video?.