iFixit

We see some fairly large speaker enclosures (well, for a monitor) near the side edges of the Thunderbolt Display and eagerly remove the screws holding them in place.

Wait! What have we here? Is that a built-in 1-inch subwoofer, as well?

The Thunderbolt Display comes with a 49 watt 2.1-speaker sound system, including a miniature subwoofer.

Basically, your display will sound something like this. Okay, so maybe not, but you will get some pretty decent sound out of this bad boy.