The Tibetan Trumpet is the seventy-second treasure that can be found in Uncharted 2: Among Thieves.
The Tibetan Trumpet is found in Chapter 20 - Cat and Mouse. This treasure is easy enough to find, since you'll quite literally stumble over it as you move forward. Once you've crossed the chasm with the tank smashing the wall above you, you'll find yourself on the roof of a manmade stone structure. Move along to the next building from there. It should be a drop down for you, with some wood jutting out from the next building. The treasure is down here, and when you drop down, the treasure icon should appear on-screen. As long as you're thorough, this treasure is virtually impossible to miss.