Looks like a miniature trumpet. If it works the same way, it could be very loud, but you'd need a little practice to make it work, and you'd have to be healthy enough to yell to use it. If you were a musician and your "chops" were in good shape, you could blow it for hours at a time, but if not, you'd be limited to short bursts. (Used to play trombone in a former life.)

Now I'm wondering if I'd stand out if I took an old trombone out as a survival signal.

In my experience, the musical instrument that carries the farthest is bagpipes. However, this would be a bad choice for a rescue signal, as nobody would ever walk toward them.