Andy,
Something to keep in mind is what is firing, and the volume of fire.
In GURPS High-Tech the rule of thumb is a man can hear a .38 pistol shot 2 blocks away 50% of the time. This seems a little low, but perhaps that's the chance to recognise and identify a single shot, figure its range or direction. Kind of like you might not need to make a roll to see a mountain but would need one to identify specific or useful features.
If an entire unit is putting out a volume of fire and/or are using MGs, then yes, I think at you should allow an Observation/Hearing check from several kilometres away. Failure might indicate you hear gunfire but can't identify the weapons/range/direction.
Tony
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