I'm a little jaded about the study.
If you give someone a suggestion ahead of time, they'll likely hear just about anything.
But I'm also sure there's some truth to it.
I "hear" things whether I'm on caffeine or not- especially if I'm run down and fatigued or sleep deprived. But I realize it's just my mind filling in blanks to make sense of "white noise" or partial sounds.
So, I think there are too many variables in the study to proclaim it all to be related to caffeine alone.