In our aviation headsets, the ‘stereo - mono’ switch (marked S – M) allows each earphone speaker to operate on a separate channel when the switch is in the ‘stereo’ position, and switches both earphone speakers on to the one ‘mono’ channel when in the ‘mono’ position.
When the switch is in the ‘M’ mono position, both earphone speakers will emit sound regardless of whether the input to the headset is from a mono or stereo source.
When the switch is in the ‘S’ stereo position for stereophonic sound, sound will be emitted by both earphone speakers if there is a 2 channel input from a stereo source, but only one earphone speaker will emit sound if the sound source is a single channel mono source.