Revealing mistake: At Fenway, when Ray sees the scoreboard cycling through images, watch the crowd below the scoreboard. They jump as if the scene cuts or is looped.
Revealing mistake: The time on the scoreboard clock when Ray hears the "go the distance" message changes from 10:30 to 10:32 in a matter of seconds.
Answer: It was actually his own voice (Kevin Costner) that sent him on the journey. At the end of the movie there's even the part where Ray is recalling the voices he heard and turned to Shoeless Joe Jackson and says, "It was you". At which point Joe Jackson turns around and says, "No Ray. It was you".