Wow, you guys need to research a bit of this. Here is a quick and easy way to figure all of this out.
1. Get in an old chevette with no muffler and an automatic. Pull on the emergency brake, stomp on the brake pedal. Rev the engine up just till the rpms wont go any higher, and relax the gas until it begins to just start dropping rpm. Listen to the sound. Now floor that sloppy little four cylinder and listen to the sound it makes. The RPMs did not increase, yet the sound is much much louder.
2. Get an old chevette with no muffler and a stick shift. Get her up to her top speed (about 20 mph) on a flat road. Put it in 3rd gear and keep her at 20 mph. Listen to the sound. Check the rpms, we will assume they are at 1500 rpm. Now, down shift into 2nd and get her back to 1500 rpm and keep it at 1500 rpm (not 20mph). The engine will make the same sound as long as you are not accelerating. Now press the throttle half way down. Dont move it, just shift gears as you speed up. You will notice that the volume does not change, just the pitch.
3. WEP does not necessarily do anything to your rpm. In a constant speed prop, your pitch will increase to take a bigger bite, cause you have more power, and you can keep the prop at the speed you set. All it does is create a denser charge in the cylinder (at least for nos) and thus more oxygen to burn the fuel.
Now the only bug I see is the sound problem in the mustang. Personally I think it should just scream when you turn on the wep. Even if not realistic it would be cooooooooool.