[Legbreaker]

Yes, it really is as long as a peice of string.

As has been pointed out, wind, rain, terrain, background noise and about a billion other factors are in play. However, the larger the calibre of the weapon, the greater the volume (usually).

For example, a 7.62mm x 51 gunshot can be heard very well up to a couple of miles in average conditions (ie countryside with normal sounds) while a 5.56mm might be just half that.

The actual sound of the projectile ripping through the air on the other hand is something else (remember, most military rounds are supersonic and have a supersonic "boom"). When a shot is fired at, or at a target nearby, it is relatively easy to hear, and with minimal training estimate to a reasonable accuracy the range, and even general location of the shooter (in a similar way that distance to lightning can be estimated).

For your example of a 15 x 4km island, I'd say most of the enemy forces can either hear, or will soon know about the gunfire (I presume there's locals living on the island too?). Single shots might be ignored (provided they're the only one fired within half an hour or so and could be passed off as a car backfiring for example), but bursts are DEFINATELY going to get noticed.

The best chance of the PCs remaining unnoticed for some time is to a) avoid combat (or better yet contact of any type), or failing that, use subsonic rounds from preferably silenced weapons (or just knives, axes, fists, harsh language, etc). They insist on assault rifles and up and they're going to be heard.