Tom, these experiments did not use RE interpretation for the results. When seismic waves are detected in locations around the world they are precisely recorded. The time waves take to travel to location in RE and FE are different because you stretched the whole Earth around. 
Ask any Seismologist what shape he assumes the earth to be when interpreting his results. Guess what he would say.
Using this assumption, and basic figures for the radius of the round earth, he can time the response of earthquake waves and very easily determine the speed of waves with simple math. If instead the waves are traveling through much more, or less of the earth than assumed in his model, it affects the velocity estimates for various types of earthquake waves. Therefore, the fundamental speed of wave propagation is dependent on the very shape of the earth.
To recapitulate, the Seismologist is making a number of assumptions when interpreting the arrival of distant earthquake waves. Furthermore, the seismologist is assuming a certain density those waves are passing through. He is assuming densities for areas no man has ever seen or explored. Man has never penetrated the crust of the earth.
Not only that, but he is assuming a certain depth to the core. The Flat Earth model's depth to the equivalent core layer is unknown. So if we push the core deeper than the core in the Round Earth model the waves will take longer to propagate into the earth and bounce back to the surface.
Consider this example: Alaska detects P and S waves which extend to North America and Southern South America. With the stations detecting the waves equally apart, the wave should take about twice as long to reach the furthest point than the intermediate point. Yet it detects it in the southern region much sooner though still after the middle region. This is observed in all directions all over the Earth by countless scientists, geologists, hobbyists, etc.
It doesn't matter how long the waves take because those waves were interpreted based on an incorrect Round Earth model. If seismic waves were interpreted on the correct Flat Earth model we can conclude very different results.