Go back into the room with the seated statue. Don't get too close to the painted sarcophagi on the north and east sides of the room, or they will open, releasing 2 mummies. You'll then have to do the next bit of business with the mummies slugging away at Lara. Unless you have cheated and unlocked explosives, the mummies can't be killed, so don't waste your time and ammo.

Notice the two floor tiles with blue-and-red borders and red circles in their centers. Push the seated statue from one of these to the other, like so:

From the entrance, approach the statue and hold Action to grab onto it.

Continue to hold Action and briefly press Forward/Up to push the statue forward a little. As you do this, the sarcophagi open, but the mummies won't emerge as long as you don't get too close.

Move around to the right side of the statue...

...and push it again....

...onto the other tile with the circle next to the empty sarcophagus.

A brief cutscene shows a door opening behind the mummy on the east side of the room.

Approach this mummy to activate it. Then hop back a few times to lure it out of its alcove.

Run around it and go through the door in the back of the sarcophagus.

Follow the passageway and drop down. Mummies can't climb, so you're safe now. Continue to a large cavern.

[Burial Chambers Walkthrough]