When your internal engine is on, it severely dampens exterior noises, including enemy engines. This is also true even if you turn your sound slider for internal engine WAAAAY down to zero.
You can show this by turning your internal engine sounds off, and take off in a plane. If you kill your engine with the E key all of a sudden you hear a LOT of things that were almost silent before. The second you start the engine up again, the sounds go away.
Even if your internal sounds are down, you gain no real advantage because the very presence of the sound (regardless of the volume) drowns out others.
P.S. It's not just a rear angle issue. You pass a formation of bombers and you'll hear the dull roar. You merge nose to nose with an enemy plane and you'll hear something.
I don't think it's realistic to expect that these types of planes with these types of engines are going to be undetected by anybody. When I mow my lawn with a gas mower it's very loud, but I can still faintly hear the guy down the street mowing with his gas mower, as well.
IMO making the game not play sounds so that people can sneak up on others is gamey. Your mileage may vary.