Label 3 points to the frog's eardrum. The ear structures function much as ours. The eardrum of most frogs is a membrane surrounded by a cartilage ring. Sound waves vibrate the eardrum, which wiggles a rod connected to the eardrum, which sloshes fluid in the inner ear, which waves hairs in hair cells. The hair cells contact nerve fibers, which generate electrical pulses.
Nerve fibers carry the signals to the brain, which interprets the nerve signals as sound.
That's how the ear structures work. The lungs are a different story. Suppose a tree falls in the night. And suppose a frog squats within hearing range of the noise. Then, just as we discussed, his eardrums vibrate in response to the noise.