First, the critter breathes in air through its nose while its mouth is closed. This air goes down into the lungs. The sac inflates, like blowing a bubble with gum. The frog or toad then pushes the air out of the sac over the vocal cords to make a call, and the sac deflates. The sac really does not make the sound any louder, though.
Some amphibians have one sac, like a toad.
Others, like a leopard frog, have two sacs, one on each side of the throat. In Pennsylvania, usually only the males call. They call to attract a mate, defend territory, or warn of danger. The sound each species makes is different from other species. With a lot of practice, you can identify which kind of frog or toad you're hearing, just by knowing their calls.