In order to cross the pit, shoot one or both of the lion's head decorations high on the wall to fill the pit with sand.
To do this, press Escape go to inventory and choose either the revolver or crossbow. Select 'Combine' and use Left/Right arrows to select the laser sight, which you should have picked up in the room with all the jars. Press Enter to combine the weapon and sight. Select 'Equip' and press Enter again to draw the weapon.
Hold Look to switch to laser-sight view, Duck/Crouch and Dash/Sprint buttons to zoom in and out, and direction buttons to aim. Press Action to fire as usual. Then release Look to return to normal view.
When you hit either lion head, the pit begins to fill with sand, the mummies on the other side awaken.
You can shoot both lion's heads if you like, but it's not necessary.
NOTE: Lara will auto-aim at the mummies, but unless you've used cheats to unlock grenades or explosive crossbow ammo, you can't harm them, so don't bother trying.
When the pit is almost full of sand, run across, run/jump past the mummies, and continue up the stairs along the right side.
As you go, a spiked ball comes rolling down toward you. Veer toward the burning torch on the left side of the stairs to avoid being skewered.
After the ball passes, continue upstairs...
...and run into the red-lit alcove on the left to avoid a second spiked ball.
Continue to the room at the top. The gate closes behind you, so the mummies can't follow.