Tanks may be loud, but I seriously doubt you'd hear them in a fighter at 1,000 feet, with the canopy closed, and the motor running. I also seriously doubt that you'd kill the engine while on a recon flight to listen for the crack of rifles or the rumble of vehicles. While we do a lot of things in AH you wouldn't dream of doing in RL in combat, some of the things are arguably "gamier" than others.
To play DA on this, I can also see some, eh, somewhat logical, I guess, reasons for being able to do so. We don't have visible disturbance of trees and shrubs to giveaway position, GVs don't leave tracks in the grass/sand/dirt, you can't see a pall of dust and smoke from an advancing group of tanks, no spotters, etc. We have sound, flashing icons on a map, identifying icons on planes and vehicles, and known points where they will appear.
I agree that it seems kind of gamey, but my guess is that it either would require some substantial changes to how sound is handled, or is intentionally left in place as a compromise or concession, allowing us to figure out (through a simple action that isn't entirely without risk) things that we'd learn of through radio chatter, spying, intelligence, spotters, visual cues, etc.
