How do I make volume changing depending on the distance?

Hello,

I have a sound effect called "FX" that plays when a flame spawn from a motionless gun.

I have a player sprite called "Player" that can move around the map.

How do I make the "FX" volume changing when the distance between the player and the motionless gun increases/decreases, so that "FX" can be heard from the player just when "Player" is near to the motionless gun?

basically set volume to distance(player,gun) but since the distance can be very high or low values try things like 1/distance, or 1-1/distance, 0-distance, etc