I know its an old post but in case someone needs the answer. Click on the sound object on your slide. A Playback ribbon tab will show up. Go to it and click on Trim Audio.

Thanks. But after trimming the audio to end within a fraction of a second, the final video still has the hanging silence.
The problem occurs on every video so that regardless of how finely trimmed the sound bars are, there's still hanging silence before the next section starts.

I see. that does make it difficult. My guess is that PowerPoint is not going to edit anything but 1 second increments. For this task you would be best to use Audacity or another DAW. Fix up the audio to have no silence then insert back into PowerPoint.