Here's one of my theories on Red John's voice and which might explain a few other things as well.

There are three things that have not been explained yet:
1) Why does Red John's voice sound the way it does, when the only two characters from the list with a similar sounding voice have been killed
2) How was Red John talking on his phone at the same time he was talking to Patrick in S6E1 (he can't have been using bluetooth or anything because he was using Teresa's phone, but could have had it propped up against his head with his shoulder)?
3) What's so important about Bob Kirkland's brother?

My idea is that Red John has an accomplice, one that he uses to speak to Jane for him. This would be someone he met long ago, like Bob Kirkland's brother. If we assume that Bob has close to the same voice as his brother, then this is a possibility. This would explain how Red John was talking to Jane while using both hands to draw on Teresa, since he wasn't actually the one talking. Bob Kirkland's brother must be important somewhere. Red John also made Lorelei Martins read the message to Jane instead of reading it himself. BH mentioned how there was an important clue in the first episode of the season that he was sure we wouldn't get until the reveal. I think the pigeons, 666, etc were all meant to distract us. Maybe this was the clue? Also, Red John did talk directly to Jane during the fake red john-wannabe episode finale (season 2?). Maybe Red John sent in Bob's brother for this. It didn't seem like Red John to be carrying a gun around and shooting people.

Leave a comment to disprove my theory, it's more useful than just an 'unlikely' :)