The Skull Cap Drum is the fifty-eighth treasure that can be found in Uncharted 2: Among Thieves.
The Skull Cap Drum is found in Chapter 17 - Mountaneering. When you reach the end of the river of flowing water careening down the ice, you'll see a small waterfall where the water drops off of a cliff to an area below. To continue onward with the chapter, you're going to need to head leftward. But first, head rightward from the waterfall. This little alcove contains the treasure you seek, and is easy enough to obtain. Just keep an eye for the on-screen prompt letting you know a treasure is nearby, since seeing the treasure's glint in the ice is a difficult proposition.