An answer to my question - why don't priests sing?
"Perhaps it's a habit, developed in years of wanting to spare the congregation their miked but imperfect voice."
And I'm sure that's true. Which is why I am not a proponent of the type of microphones that make the priest sound like he is speaking from a location one micron away from your ear drum. I think that if sound system output is at a medium level, it makes people have to concentrate just a bit more to listen. And it's nice to be able to get away from the mic or turn off the mic if you'd rather not be heard singing, telling the deacon his homily was too long, or asking the altar boy to please get the sacramentary instead the binder all the announcements are in. "Tommy - I need the big red book. The red one. Over there. Gold ribbons. Tommy - not over where the lector talks - the little table... c'mon Tommy, turn around and go to the little table..."