Answer
The mountain is 4900 km away in this hypothetical situation.
Work Step by Step
During the 8 hour time interval, the sound wave went to the mountain, and back. The sound reached the mountain in 4 hours, or (4 h)(3600 s/h) = 14400 seconds.
$$d = speed \times time = 340 \frac{m}{s} \times (14400 s) = 4896000 m$$
In reality, the energy in the sound wave would have dissipated relatively rapidly as the wave spread out in 3 dimensions. Rip would not have heard the echo 8 hours later.
