The Tibetan Conch Horn is the eighty-third treasure that can be found in Uncharted 2: Among Thieves.
The Tibetan Conch Horn is found in Chapter 23 - Reunion. located in the area with the tree on top of a flight of stair (There are also enemies being attacked by the guardian), it's best to go for this item after the fighting. If you are standing at the tree at the top of the stairs, with your back to the tree you will see 2 gold statues on the top right building. You will see a pillar in the middle below them. The treasure is on the right side of that pillar.