Soft, uneven surfaces absorb sound waves like a sponge. In a room full of bodies, there are no hard surfaces left for sound to bounce off of!

While the idea of a room full of dead bodies sounds like something out of a horror movie, the reason you can't hear an echo there is actually pure physics. It's the same reason a recording studio has foam on the walls or why an empty house feels much louder than a furnished one.
To create an echo, sound waves need to travel from your mouth, hit a hard, flat, and smooth surface (like a concrete wall), and bounce back to your ears. If the sound can't bounce, there is no echo.
Human bodies (living or dead) are not hard or flat. They are made of soft tissues, skin, and clothing. In acoustics, these are called absorbers.
In a room full of living people, you also won't hear much of an echo. However, the "dead body" version of this fact is often used to highlight the extreme silence of a morgue or a tomb. If a room is packed from wall to wall with soft objects - whether they are mattresses, heavy coats, or bodies - the room becomes "acoustically dead."
An echo requires a "mirror" for sound. Bodies act like "blankets" that soak up sound waves. If a room is full of them, the sound has nowhere to bounce, making the room feel eerily quiet and muffled.