Is there any truth to the old...

This phenomenon may be true but not because cows don’t like thunder (although they might not). Hot, humid weather, common on summer nights, causes unstable atmospheric conditions that can lead to thunderstorms and lightning. These same conditions also are responsible for warming milk and triggering the release of microorganisms that can lead to the souring process. So, in a way, you could say that thunder sours the milk, but we don’t see a direct cause and effect here. Try it yourself. Leave a glass of milk on the back porch during a thunderstorm and see how long it takes to congeal.