Here is some explanation of Red John's voice.

"In season two's "Red Sky in the Morning," the first episode where Red
John physically appeared, Patrick Jane was wrapped in cling-film and his
archenemy recited William Blake's "The Tyger."

"Baker did Red John's voice and it stuck," Heller said"

http://www.today.com/slideshow/today/the-mentalist-53592223

then go to slide 5.

First time I have read anything that confirms who did Red John's Voice!!