I don't know the person's name, but the person replacing Mako is someone who studied underneath him. They say he sounds exactly like Mako did, but I beg to differ. There were a few snippets of the replacement voice in "The Tales of Ba Sing Se" during Iroh's tale during the soccer scene and in "The Earth King" when Zuko was sick and Iroh was telling him to drink some water. I found it obvious that it was a different voice, but I must admit that it could have been much worse and they did find a good replacement.