Here is an interesting experiment worth trying: Stand before a glass and pucker up your mouth and whistle, and note the shape of your mouth and the general expression of your face.

Then sing or speak as you do naturally, and see the difference.

Then start to whistle again for a few seconds, and then, _without changing the position of your lips or face_, sing a few notes and notice what a vibrant, resonant, clear and beautiful tone is produced..