Underscoring as a cinematic ground layer for the scene. What happens when something turns cinematic? What does that turn describe?
Underscore verges on soundtrack. Maybe a designation of their difference is that underscoring moves a little lower, not calling itself to the listener-watcher’s attention so much as sculpting the mood, whereas soundtrack (think the intrusion of popular music) makes a grab for straight for the feelings, nominates itself as the emotional frame through which to mark the moment.
Both of these things as frame. The frame needed to mark off a moment so that it can be read for its meaning instead of merely pass us by or carry us along.