At the end of this episode Murphy is floating on his back in the pool with only his anterior face out of the water; both ears submerged. His neighbor is speaking on normal conversational tone and volume and Shawn responds. With his ears submerged he wouldn't have been able to hear.
When Kenny, Shaun's new neighbor, lets Shaun into the pool, the sign on the inside says "closed" clearly indicating that the other side of the sign says "open." Kenny wouldn't have had to pick the lock to let Shaun in.