Frogs Make Sounds and Hear in Surprising Ways

Frog Sounds and Listening
As warm weather is setting in the chorus of frogs is starting up once again. Every time I pass a pond or water filled ditch I'm amazed at the cacophony. One pond I like to visit is devoid of any man-made lights or nearby sounds, so when you close your eyes and listen, just sitting in the dark is like being in a sensory deprivation chamber of just frog calls, quite stirring. This all got me to thinking, how do frogs make sound? Also, since they don't have external ears how do they hear those sounds?