Fast trigger volumes

Zones.lua

local RunService = game:GetService("RunService")

-- compile an oriented bounding box into a scaled CFrame
local function compileBBox(cframe: CFrame, size: Vector3)
	return CFrame.fromMatrix(
		cframe.Position,
		cframe.XVector/size.X,
		cframe.YVector/size.Y,
		cframe.ZVector/size.Z
	):Inverse()
end

-- point-obb occupancy test
local function testPointBBox(pt, bbox)
	local objPos = bbox*pt
	return
		math.abs(objPos.X) < 0.5 and
		math.abs(objPos.Y) < 0.5 and
		math.abs(objPos.Z) < 0.5
end

local function initZone(zoneRoot, pointCallback, enterCallback, exitCallback)
	local bboxData = zoneRoot:GetChildren()
	for idx, part in bboxData do
		bboxData[idx] = compileBBox(part.CFrame, part.Size)
	end

	-- check bounds to select initial callback
	local lastCallback = exitCallback do
		local pt = pointCallback()

		for _, bbox in bboxData do
			if testPointBBox(pt, bbox) then
				lastCallback = enterCallback
				break
			end
		end
	end

	lastCallback()

	-- use heartbeat to run after physics solve, prior to render
	return RunService.Heartbeat:Connect(function()
		local pt = pointCallback()

		if not pt or pt ~= pt then
			return -- ignore nan, nil
		end

		local sel = exitCallback

		-- select a callback
		for _, bbox in bboxData do
			if testPointBBox(pt, bbox) then
				sel = enterCallback
				break
			end
		end

		-- dedup and run callback
		if sel ~= lastCallback then
			lastCallback = sel
			sel()
		end
	end)
end

-- usage

initZone(
	game:GetService("ReplicatedStorage").SoundZone1, -- folder of parts
	function() -- character's position
		return workspace.CurrentCamera.Focus.Position
	end,
	function() -- entry callback
		print("entered SoundZone1")
	end,
	function() -- exit callback
		print("exited SoundZone1")
	end,
)