Replace ambient sounds from unknown sources with CC0 licensed ones

Sample sources:
rain https://www.freesound.org/people/vibe_crc/sounds/50058/
thunder1 https://www.freesound.org/people/netaj/sounds/193170/
thunder2 thunder-ambient
https://www.freesound.org/people/hifijohn/sounds/242735/
thunder3 https://www.freesound.org/people/sarson/sounds/195522/
thunder4 https://www.freesound.org/people/FenrirFangs/sounds/234737/
thunder5 https://www.freesound.org/people/Xythe/sounds/37299/

mods/ra/maps/fort-lonestar/fort-lonestar.lua

@@ -152,9 +152,12 @@ Tick = function()
 		local delay = Utils.RandomInteger(1, 10)
 		Effect.Flash("LightningStrike", delay)
 		Trigger.AfterDelay(delay, function()
-			Media.PlaySound("thunder.aud")
+			Media.PlaySound("thunder" .. Utils.RandomInteger(1,6) .. ".aud")
 		end)
 	end
+	if (Utils.RandomInteger(1, 200) == 10) then
+		Media.PlaySound("thunder-ambient.aud")
+	end
 end
 
 WorldLoaded = function()