Spooks extract - sound

Though sound is used subtly in this extract it can have a significent impact on the viewing audience in the same way that it displays the atmosphere in room without focusing on the light or camera angles.
For example the constant undertone murmor of non-diegetic instantly gives an impression of the dark atmosphere without referring to the spoken dialogue.

The constant echo gives a clue that they are in an enclosed room this is a choice by the director to ensure that the viwer understands the danger of the situation; other uses of non-diegetic sound include what sounds to be a lightning strike when the camera zooms in on the photo of the victim.
For someone who is only listening to the clip, this noise will seem disorientating and alien.