Mitsu's Merlin sounds are authentic, but are ancient, so they're not of the highest fidelity... However, to answer your question. No, the pilot hears a much different sound in the cockpit. It's quite the same as listening to a car on the outside, and then getting inside.
Except an aircraft engine is infinitely louder. As you know, I've got the cockpit engine sounds in my packs, and they're actual recordings of each aircraft. Just rename the EngI file to Eng (be sure to back up the original Eng), and you're set. Also make sure to do the same for the startup/shutdown sounds, as well as every other sound that has an Internal counterpart.
