Many choristers fail to open their mouths enough to create a lovely sound.

To fix that assist them in being able to feel the physical position that is required, start by writing on the board:

Oh
Eee
Ahhh
Oh
Ahh
Eee
Ay
Oh


Then ask them to say them from top to bottom.

Then ask them to say them without moving their lips or jaw. Reward the one who does this the best by praising this singer, have everyone turn and watch his singer demonstrate.

Ask the entire group to do it again, paying very close attention to how their jaw and lips feel.

Then ask then to say the vowels in order again, way over-emphasizing them.

Once they are able and willing to do this, then ask them to sing, without moving the jaw or lips:

Oh
Eee
Ahhh
Oh
Ahh
Eee
Ay
Oh

Sing this to the tune of Do Re Mi, but starting from the top down.

Reward them again for being able to follow your instructions.

Then have them sing them again but doing the over-emphasized vowels…vowels only, no consonants.

Then have them sing it like they normally do.

They WILL feel a difference. Get them to tell you what the difference is in how it feels, how their jaw feels and lips move.

Once they are comfortable doing this, have them sing it, from the top down in different keys, using the consonants, making sure that they are inserting the consonants between the vowel sounds and not letting the vowel sounds be changed from their pure state.

Singing a chant then without consonants will dramatically improve the musicality of their sound when the consonants are added to it.

Work out a sign, a gesture that you will use (a fist opening?) to cue them to reinforce the open sound when they begin to fall back into their old habits.

Why call it The Charlie McCarthy School of Tone Production? Edgar Bergen had to have perfect diction to be able to do ventriloquism. Otherwise there would be little or not difference between his speaking voice and the dummy's voice...Bergen's speaking voice probably fell somewhere in between, as that of most of your singers when singing.

Get them all to understand that consonants are a bump in the smooth road of vowels.

Singing this exercise from the top down will also improve their sound.