Imagine an old hermit who lives in the mountains. just before going to sleep, he yells "wake up." the sound echoes off the nearest mountain and returns 8.2 h later.

The sound took 4.1 hours to reach the mountain, then after bouncing (echoing, reflecting) off of the side of the mountain, it took another 4.1 hours to return to the old, decrepit, elderly sleeping hermit.

The speed of sound in atmosphere with standard conditions is 343 m/s.

(4.1 hours) x (3600 seconds/hour) =  14,760 seconds

The distance covered by the sound in each direction is

(14,760 sec) x (343 m/s) = 5,062,680 meters  

(about 5,063 kilometers, or 3,145 miles;  no two points in the contiguous 48 states of the USA are that far apart.)

It's important to me to point out that in order for the story to work, it's not really necessary for the old hermit to be old, or to be a hermit.