you don’t need a script, just create part with the sound in it, and set the max roll off distance of the sound to the radius of each zone. if you want a script you could just tween the volume of the sound when the player touches a invisible part surrounding each zone.
As @FragmentFour stated, use a single Part centered in each zone.
Play with both the MaxRollOffDistance as well as the MinRollOffDistance to get what you want.
Say your entire zone is 500 studs across. Put your Sound Part in the middle and set the min roll off to something like 240 studs (slightly less than half the zone size) and the max roll off to something like 260 studs so when the player walks into that zone it gets louder over a 20 stud distance.