Well a muscle car, hmm, let's see how many models of muscle cars had an option for fresh air, 70 SS Malib LS6 and LS7 with optional Cowl Induction Hood, 80 and 81 Z28, 70 Pontiac GTO Ram Air, 74 Trans Am Ram Air, Several Fords with the Cobra Jet and a special air cleaner assembly, 68-70 Hurst Olds, and then Chrysler had several models that either had some type of scoop or a special air cleaner. All these besides a few more I can't remember showed a higher HP rating than a lesser model of that same car. Now from what I remember most builders would talk about cooler denser air for an engine is what you want to have and any means to get that.
Now lets turn the clock to today's cars, don't you think these cars can benefit from a cooler denser air charge? Yes and the added benefit is a slight improvement on MPG. Factory air intake systems are not smooth, they incorporate convoluted tubes and restrictions as well as a silencer, besides the small air inlet at the air filter box itself.
Now as far as noise goes, you only hear it if you have the throttle opened up pretty far and if you drive it in a normal fashion you really don't hear a CAI much.
So with a CAI you improve air flow to the throttle body over stock air intake, you get a little more in HP over stock and you can also get a little bit more MPG, even a small amount makes a difference to some.