Earplugs is a feature within the mission, which, when enabled, reduces the surrounding sound level. This helps, e.g., in combat situations where extreme fire (e.g. from tanks or artillery) affects understanding your teammates in the voice chat (VON).
Enabling and disabling (toggling) the earplugs in King of the Hill is possible by pressing the
F1 key on your keyboard.
When Earplugs are inserted/enabled, there's an icon visible in the player's HUD (left middle side of the screen).