They're not the same type.

code/__HELPERS/game.dm

@@ -193,15 +193,15 @@
 				listeners += M
 				continue
 
-			listeners |= M
+			listeners += M
 			listeners = recursive_mob_check(A, listeners, 3, 1, 0, 1)
 		else if(istype(A, /obj/item/device/radio))
-			listeners |= A
+			listeners += A
 
 	// Don't include if the player is not connected.
 	for (var/mob/Mob in hear)
 		if (isnull(Mob.client))
-			listeners ^= Mob
+			listeners -= Mob
 
 	return listeners