mizzie, when that happens it's more often a sound level issue than the fault of the performers. if they aren't enunciating, that's their fault, but balancing sound for a live, fully miked musical is extremely hard and far from an exact science. i do hate it when two voices just blend badly, but a good vocalist should be able to blend as well as sing solo.
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Say a prayer and induce insincere self-abuse,
till I'm fast asleep"- In Trousers