When I was about 8 years old, my mother had bought one of those tape voice recorders, (this was in 2000 so younger people may not know that voice recorders required cassette tapes as there weren't really digital ones on market). It was around maybe 7:30 at night, me, my big sister, mom and my two younger twin siblings (1 at the time) were sitting in the living room. My dad had just gone to get a shower (this fact comes into play later). Being 8 years old, I grabbed the tape recorder wanting to play with it. You know how kids like playing with stuff like that. So I got the tape recorder to play with and start saying funny things and making noises into it like someone that age would.
After about my forth time recording myself being silly, I rewound to tape to listen to the play back. Everything sounded the way it was supposed to, me being silly, the twins making the noises of babies, the faint sound from the T.V in the back ground etc. Then near the end of the recording my sister being a typical big sister says to me "will you quit sounding so stupid?" If you have older siblings you know how it is.
But the thing that caught me so off guard about her saying that is right after she says it, there is a very deep, gruff older mans voice asking, "Who are you?". This voice was the clearest voice on the tape, what now would be classified as a grade A+ EVP.
Of course this spooked me. I told everyone in the room what I heard. Go figure, they didn't believe me. So I played it for my mom and sister. My sister said "that was you doing that." Remember I was 8, so if I had made that voice, we need to check the hormones in the food! My mom told me that was just my dad and to leave it alone! At the time I didn't realize the forceful nature of how she said it.
It was at that time, my dad came walking through the living room. Being the stubborn child I was and ignoring what my mom told me I ask him-
"Dad"
"what"
"Did you ask "who are you?" about five minutes ago?"
"Why the hell would I ask that? Besides, I just got out the shower."
That's when my mom got mad and told me to stop again.
Unfortunately, that tape is lost to time now. Being that young and not knowing much about the paranormal, it never occurred to me to keep it. It's this experience that I credit the most as to what got me so deeply interested in ghost and the paranormal in general.
[As a side note, years later I've learned of experiences she had in that house before I was born that left her traumatized and is the reason she told me to leave it alone. I'll ask her if its ok to share them.]
I was thinking more along the lines of whenever a ghost "speaks", for lack of a better term, they emit a small burst of energy similar to an EMP. So like how our voices vibrate the air and in turn those vibrations are translated to the tape via the recorder, a ghost's "voice" would be directly recorded. It could explain why we can't hear them since our hearing is based on interpreting sound waves and not magnetic pulses, but they can be recorded.
Of course this is all just speculation. There's still so much we don't know about ghosts to draw any kind of conclusion to.