just some rough ideas...
Artillery can be heard as a distant rumble at about 20 miles or such, can easily be mistaken for thunder except for the regular pace of the firing.
Small arms can generally be heard at about 2-3 miles. You can almost always tell when a unit was on the firing range for example.
Of course wind and wind direction play major roles as well as terrain, even trees can block the sound of firing at a distance.
This will sound odd, but when tanks move you normally feel the vibes long before you hear the engines, and if the tank is fitted with a turbine, then the squealing of the tracks can be heard at about 300-400 meters. If the tank is fitted with a diesel engine, the key indicator for movement was to watch for the dark puffs of exhaust as the tank moved uphill, depending on the optics being used this could be spotted as much as a kilometer (using binos) or 2-3 kilometers (gunsights flipped to x10).
hope this helps!
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