I really like this film about Johnny Cash. J. Phoenix did a great job of singing along with Reese. Phoenix had Cash's mannerisms down perfectly including how he held the guitar. At first I thought they dubbed in Cash's voice but in the credits it stated that Phoenix did the singing.
shareHey Slimone - I know this post is 3 years old but just wanted to comment that I'm pretty sure Cash's real voice was "mixed" with Phoenix's for the movie.
I have seen this movie probably 15 times (7 times in the theatre alone!) and I thought the same thing as you when I first saw it (Roger Ebert agreed - in his review he said when he closed his eyes while watching the movie he couldn't tell the difference between Phoenix and Cash's voice) but Phoenix simply sounds WAY too similar to Cash to be all him.
This was confirmed (for me at least) by listening to he official soundtrack - Phoenix's voice sounds MUCH different from how it sounds in the actual movie. To be honest, if they did mix in Cash's voice I'm kind of glad they did as it really adds to the atmosphere/believability of the movie, but wish they had of kind of acknowledged it at some point (in the credits maybe...?) I always wondered about the legality of such mixing...