It could be the Sage of Six Paths. Since Naruto has the power of the Nine - Tails and is on a path to becoming a great ninja, the Sage might be trying to communicate with him through voices, guiding him to fulfill some greater destiny.
It could be his parents. Minato and Kushina might be trying to communicate with him from the afterlife. They could be sharing family secrets or just giving him words of love and encouragement.