Mary begins by telling God that she praises Him from the depths of her soul. I learned a long time ago, when I was minoring in music in college, that there are different ways that we can sing with our voices.
A person can sing in their throats. That’s not recommended because it tends to be a little off key and it wears out the voice, perhaps even produces a sore throat before long.
A person can also sing using their diaphragm. Using the muscles surrounding the stomach gives a deeper, richer sound to a song and doesn’t tire out the vocal folds in the throat. It also allows the singer to hold long notes without running out of breath because they can more easily control their diaphragm muscles. This is the ideal way to sing.
Mary sings from her soul, not her lips, not her diaphragm but from the depths of her emotional being.