*Blinks* I am assuming your not talking about audible voices. There is nothing wrong with this of course, it happens to people all the time. But if it were the case, I suggest you seek a psychiatrist to talk with, even if it were demonic in influence (and it probably isn't); you need someone to "ground" you and help you cope with it. Ultimately, psychological issues are more likely, and it doesn't mean that you're crazy by any stretch (
and that doesn't necessarily mean you'll have to take meds or that it has to be 'bad'). (EDIT: A note for the future, to avoid further confusion, if you're not talking about audible voices, you can refer to it as "internal dialogue" or the kind of experience you have in reading a book silently)
That stated; try though I might, I can't hear God in my head either - and I doubt many of us here truly can. I've always wished for a telephone to God, it would be the most awesome telephone in the world. But I'm just not spiritually tuned to picking up spiritual messages all that often - I could tune out the whole world, and I would just be left with psychological noise - no better then listening to white noise to "hear God". In fact, scratch that, with ADD - it's impossible for me to "tune out" my reality - my brain creates entire fantasy worlds the instant I close my eyes ^_^ (In fact, I spend probably less then 1% of my life actually "looking" at the real world, the rest of the time I'm chillin in my own mind XD).
In life, I've only deeply gotten a message from God twice - both instances vastly changed my life from that day onward. Maybe that's why he doesn't send more - it would overload my life with transformation and I'd spin out of control from it all.
So in the end, don't worry too much - if we're talking about your ego and the million little influences (often contradicting each other) in your life, it's pretty common. I don't think God is limited in his volume setting, so I suppose he wants me to listen to my own heart and grow according to who I truly am inside (perhaps that's what your Grandmother was saying - not that God talked to her and had her voice, but that he formed a heart in her that told her to live a life he would be proud of). If he does speak, I am rather excited to hear what he has to say next, it will most likely change my life - although it he just wants to call to discuss his sunset last night, that would be cool too.