Prayer, seems to me, is just a pond of Confirmation Bias to fish from and it is indistinguishable from random chance.
I mean, if God really wanted to make himself known, wouldn't he choose one religion and enable them to manifest anything they need and want through prayer clearly and consistently? It would be the most wildly successful way to make himself know and clear. Man, this God lacks imagination.
At the very least, he would provide some type of reliable and testable mechanism and guidelines to understand if it has any efficacy. Again, no imagination, no thinking through the problem...epic fail.
You would think this would be the nail in the coffin for most religions and Gods. The amount of Cognitive Dissonance in the heads of people who pray must be deafening. Maybe that's the voice they're hearing.