In working with church singers, I've noticed a bit of a problem. It seems many of them have trouble projecting their voices, most particularly during the liturgy itself rather than in rehearsal. In rehearsal we go over all the vocal technique stuff: proper posture, forming the vowel first, "plopping," "tucking," getting the soft palette out of the way, etc. When all these things are remembered, a choir can sound completely different, in a good way.
But something seems to happen during the liturgy quite often, and this problem seems to be more severe with the singing of Gregorian chant. Often it seems that the beautiful sound goes out the window, and everyone is haunted by this quiet voice that says, "Shhhhhhhhh. We are in cheeeeyurch!" What I'm trying to say is that people seem to let a certain idea of piety--indeed, a caricature of it--take over their sense of what music should sound like in church. Everyone backs off, breath support dies, and the vowel sounds lose their luster. This simply can't happen. Sing out! Holding back is not going to make you sound like the Monks of Solesmes.
It seems to me that it is more important to sing well than it is to sing "piously."