Not only are most performers miked today, most musicals use pre-recorded vocal tracks to help fill things out.
If you're curious as to mike placement, look at actors' hairlines; you'll see tiny projections either where the forehead meets the hair or just over the ear -- those are the mics. (Those actors who are wigged will have a little more success in hiding things.)
Up until the 50s and 60s, actors and singers had to rely solely on their own lung power and projection, but as audiences demanded more sound, orchestrators and sound designers pumped up the volume from the band to the point where not even Ethel Merman could be heard over them. Unfortuately, even straight plays are having to resort to them, except in intimate Broadway houses such as the Booth and the Belasco.
Next time you're in a Broadway (or Broadway-size) house, look for the sound booth in the back of the orchestra or in one of the side boxes. You'll see any number of mixing boards, individual readouts for each performer's mic, and probably even a CD player or two.
All shows are miked somehow -- whether using mikes on the floor or in the flies -- so that the actors backstage and the stage managers can hear what's going on onstage. Those mikes don't carry sound into the house, though.