After obtaining the MUSIC SCROLL and returning to the upper-level balcony in the MAIN HALL, as described in the main walkthrough, barge open the right door on the upper level, north side. (This is the door marked H in the diagram.) If you've just come through door I or J, on the south side of the room, you'll go around to the far side of the balcony, as shown above.
- OR -
If you've come via the FIRE CIRCLE PUZZLE ROOM and ARCHIVES, and emerged through door G, you'll just turn left and left again to get to door H.
Shoot the vase in the hallway to get the Uzi ammo inside. Then continue to the room at the end of the hall.
Face the music stand near the harp and press Action twice to place the MUSIC SCROLL on the stand.
In a brief cutscene, Lara plays a tune on the harp and a nearby door opens.
Go through this door and follow the passageway...
...to the small balcony in the northeast corner of the MAIN HALL. Pull the chain.
This opens the big blue doors.
Take a standing jump over the railing to land on the steps below.
Then go through the doors you just opened to the next level.