Yes, we mumble on the radio at times, but so does every other nationality. Ever heard a Texan on the radio? Some are crystal clear, some sound like John Wayne.
Why do Canadians and Americans pronounce one of the LHR holds Bovington? (with a T, when it's Bovingdon with a D)
Why does JFK ATC insist on speaking so quickly that many Nationalities ask them to repeat the instruction or clearance?
Australians are not perfect, but there again, no-one else is either.