Since no one has chimed in, I'll give you my limited opinion.
Since the issue only happens at 4 specific locations on the track and the engines work flawlessly on a test track, I don't think it is a tether issue right now. I would think that you may be experiencing a temporary voltage drop at those 4 locations which is causing the sound to drop out.
I would first test the track voltage with a meter on the 2-3 sections of track at or near each location.
If that tests out, I would remove the 2-3 track sections at the "easiest-to-remove-track" location on the layout where it is happening and hardwire (solder) those sections together; re-install; and see if that eliminates the issue at that location.