I've mentioned this before on the subject of ghosts, (It should annoy everyone as guess what... it involves parallels lol)
Anyway, simply put there are many institutes the world over working on clandenstine experimentation some in the field of long range electromagnetic communication and others in more cutting edge physics fields like Super String.
What Increan refered to as EVP(Electronic Voice Phenomenon) can be manipulated, there is also the longshot that a natural occurance can occur. For instance I'm sure everybody has had a time when the light has played tricks on their eyes, or when they've started to speak to someone they think they know just to realise that it's someone else.
This kind of occurance occurs to how we store information in our minds, and how we process that stored information against an event like sight or sound.
This is also the case with the most sophisticated Neural pattern cognative systems for sound analysis so thats the natural instance explained.
The un-natural instance is due to the universities in the world that get funding for paranormal studies. If the paranormal was proven to be non-existant and stopped, so too would their funding, so they manipulate events that 3 party people hear, see or find to make sure their funding continues.
Afterall it's very simple, theres equipment to pick up sounds from a distance like a conversation a couple of hundred metres away. So there is most definitely equipment that can process the distance of an object and then calculate the speed necessary to send an electromagnetic signal that once it slows down from rebounding off the object, emmits sound.
Such equipment has been used for years by the military for searching out Submarines, in this case a sonar signal is sent out to then process how it rebounds and from where.
(I'm sure Adam will elaborate)
This voice version is just a kind of markup.
As far as my concern of ghosts... They are just another event deemed by those that don't know as un-natural or paranormal, while those that do know tend to snicker in the shadows and continue because they realise that they've got someone on a string.
The usual comment here is, "But ghost have been around for years before the technology, how can that occur?"
Well those years you didn't see yourself, and for the other instance I started this with a mention of parallels. Well there is an instance in String Theory, that when a parallel diverges into two seperate paths, it had two unique universe to start with.
Occurances like sound can travel between these universes, so hearing a chair move is either because in another universe someone moved it, or at another point in time someone did.
(I tend to look at the possibility that parallels can be shifted incrimentally in time, so 5 minutes from now could exist at the same time as now. [Useful for Quantum computer enthusiasts.]
This time version would allow an understanding of how a past or future event could be observed.)