I think this is happening to me as well. Sometimes I notice it if I take the phone away from my ear for a moment to glance at the screen, then put it back, I hear the other person very faintly. At first I thought it might just be how I was positioning the phone against my ear, because after adjusting it for a few seconds, I could hear the person loud and clear again. But it happens often enough that I suspect something might actually be going on with the hardware. I'll see if I can test this more methodically somehow.