The four Sensor boxes are the Receptive Transducers again, used to detect stimuli produced by the Expressive Transducer.
The four Sensor boxes in Mysak's model are the Receptive Transducers again, but this time serving a different purpose. They are tuned to detect the stimuli that are emanating from the expressive transducer.
That includes the sounds that are being produced, the sensations of touch from the tongue and lips, and the kinesthetic feedback from the jaw movements, to mention a few.