Blowing Horns.

Blowing horns have been in use for centuries as a way of signaling. The signal is predetermined by the people using the horn, a series of blows could send a specific message, or the very fact that the horn has been blown could signal the coming of danger, or we have game come and help carry it.
The ability to make noise lies not solely within the horn, it has no special insert within the mouth piece, the ability comes from the person blowing the horn. Some horns are carved, others are not. Some horns have a shaped mouth piece and yet others are plain. All though seem to have had the mouth piece carved out slightly to form the shape of a funnel. 

A 17th century etching of a man blowing a horn.

My blowing horn. I did not make this and can't remember where it came from.

The mouth piece on my blowing horn is rather crude and rough, but you can see the funnel shape cut from the hole.

http://www.goosebay-workshops.com/ANTIQUES-2;

Here again the funnel shape on these horns is very evident.

http://www.goosebay-workshops.com/ANTIQUES-2;

This 19th century blowing horn has been shaped and embellished, but only for aesthetic reasons I would assume, as the lips or mouth do not go over the mouth piece, only against the end. 

http://www.collectorsfirearms.com/large-blowing-horn-cur140/#.Ut8XpBB9LIU;

An old blowing horn found in a barn.

http://contemporarymakers.blogspot.com.au/2012/09/antique-blowing-horn.html;

A conch shell blowing horn.