How many times have you heard a frustrated mother say to an obstreperous tot 'Speak in your indoor voice!' This often calms the moment which was getting out of hand volume-wise.
In singing one must sing in your 'outdoor voice'. This does not mean you holler or scream, but it also does not mean you don't mumble.
The whole point of vocal performance is to be heard in some sort of hall and mumbling just doesn't cut it.
This lady knew how to sing in her outdoor voice!
In working with students who are trying to sing with their indoor voice, I emphasize using your energy to easily project the sound. Your air is your energy. Indoor voices are fine in small noisy children but don't work at the Met.
A combination of the best possible breathing technique with a sense of the distance into which you are singing will produce the correct amount of sound in any room.
To simplify: the singing breath should begin by listening to an 'aw' sound as the air passes down your throat as you inhale. Then, immediately return the air as sound. I have students bounce a tennis ball to get this timing right. Inhale as the ball hits the floor and sing as you catch it. This way you use the deep breath you have just taken to its best advantage and the whole breathing thing becomes fun and easy.
Choose a point somewhere in the distance to which you sing. You do this automatically when you speak to someone on the other side of the room. Use the image of distance to allow the voice easily to project.
Timid singers don't have careers. Be brave! Be bold! Sing with your outdoor voice.