Enemies don’t rush into battle once they’ve been discovered (they have, in fact, no idea whether they’ve been discovered or not) but once they themselves discover the player, are being attacked (which reveals the attacker to them, just as it does for the player), see their allies rushing into battle, or have vision to an ally which knows of a player position and shares that information. That is, assuming the enemy thinks that attacking is a good option given the tactical situation; a bandit wouldn’t rush to attack 6-8 men on his own, and even skeletons right now often prefer to retreat to a more defensible position or towards allies if the player has a clear numerical advantage that the skeleton knows about (again, limited vision!), or at the very least play more defensively (use shieldwall, spearwall, etc). Zombies don’t care for these kinds of considerations, though, which is one reason why they’re easy to outplay.
If I understand you right, you’d like enemies to rush towards the player from all across the map once a fight breaks, though, on the basis that they should hear the sound of battle no matter how far away they are?