Is this in fact, the technique used by a number of blues/folk singers called the "invisible trumpet/harmonica where the lips are pursed as if playing a brass instrument and the singer sings notes while opening up as much internal air-space as possible resulting in the production of a trumpet sound?

David Suchet the actor is apparently a fine exponent of this having had to learn it to appear in "Blott on the Landscape" I have also heard Eddie Walker and John Kirkpatrick performing a quick "cornet intermezzo"