Well, yes, the "White Sound Device" (or "Heartbeat Reader" as FJ called it.) But, it seems to be lying around in all kinds of surgical places, so it must have some function we don't really understand. ("Quickly! We need to mask the sound of this patient's heart during the operation!").
The James Blish novelization of "Court Martial" called it a modified 'white noise generator.' Which makes perfect sense to have during a delicate operation in a hospital setting, filtering out various environmental sounds (read: onboard ship noises) to allow McCoy and Chapel to concentrate on what they're doing and (post-op) allowing their patients to rest without the constant thrum of the ship's systems in their ears.