Why did Gary Oldman "mumble"?

Rating: 7/10 (13 February 2018)

Go to YouTube and listen to a Churchill speech. He had a very coarse voice, but he did not "mumble" as Oldman portrayed in the movie. Perhaps Oldman was unable to correctly imitate Churchill so he compensated by intentionally "mumbling". It was a good movie about a terrible time in the lives of Britain and the British people.