You're making a whole lot of presuppositions there.
Here's a question that got me well on the road to waking up and finding it was all a big lie: How does prayer actually work? (that means the mechanism, not any perceived "results")
It's a simple question, but by being totally honest about it you will find it eye opening.
I've often wondered this very thing. I've been doing a lot of thinking about this from a scientific standpoint. From a scientific standpoint, a prayer is simply a thought, one that we have directed to what we perceive to be God. If prayer is simply a thought, this can be measured somewhat by the instruments scientists have to measure such. However, scientists have yet to figure out how to intercept a thought to know its contents. We have identified certain parts of the brain that react to certain stimuli and emotions, but we can't decode thoughts. This does not mean that it is impossible though. Just like all the other discoveries in the scientific communtity that we believed were impossible only a few years ago, we are learning as human race to do incredible things. The human genome project is one of those things.
I believe that it is possible to read human thoughts, but we just haven't learned how to yet. We don't yet have the intruments or the knowledge on how to do so. But I believe we will figure this out someday (or at least people much smarter than me will ). This leads to the conclusion that if humans can figure out how to read thoughts, then God (assuming he/she/it exists) can read thoughts since he is our maker and aware of our make-up. This also would follow that Satan (under the same assumption that he/she/it exists) has much more familiarity with our human makeup than we do since he has observed us since our creation. It would be reasonable to conclude that Satan has learned how to read human thoughts as well, since it is purely a neurological metric of electrical pulses among the neurons in our brain.
If God doesn't exist, we can probably safely say, someone in the future will be able to hear our prayers (or thoughts), once we, as humans, figure out how to decipher this neurological process.