This talk was called 'Some Objections' in Chapter 2 of Lewis' book 'Mere Christianity'.
Illustrations of the Moral Law/Natural Law found in other times and cultures are taken from the appendix to the book 'The Abolition of Man' by C.S. Lewis and is available for viewing here: www.amazon.com/Abolition-Man-C-S-Lewis/dp/0060652942
You can find the book here: www.amazon.com/Mere-Christianity-C-S-Lewis/dp/0060652926
You can find the audio track here: www.amazon.com/C-S-Lewis-War-Christianity/dp/1624052185.
(This broadcast took place at the zenith of Nazi power, with the rapid invasion of Russia in full swing and hundreds of thousands of Soviet soldiers being taken prisoner.)
Correction: I discovered from old magazines that this broadcast was actually made on the 3rd of September, 1941 and was Broadcast #5 in series called 'Listener’s Questions'.