I'm not always convinced the snippets are even snippets to tell the truth PeteS, 9 out of 10 EVPs are unconvincing to me .. sort of like when you listen to a a Beatles record and people are excitedly making out 'words' that are in truth far from clear.
I'll watch a show like Ghost Adventures, they'll play back something that sounds like "Nnn skay four ears" and subtitles will be presented on screen excitedly reading 'EVP: "Run! .. escape from here!". The most convincing one I've heard so far was one an investigator caught while I was standing next to her, she was the only woman with us so I know it wasn't her speaking, her face is also recorded by a different camera at the same time, her mouth not moving and we got a crystal clear woman's voice say the word "Talk", not whispered.
Some people believe that for a spirit to talk or move an object takes a lot of energy, you know, seeing as they're dead and everything so they shouldn't have any energy scientifically speaking at all .. hence the whispered snippet theory.
A friend of mine has an endearing theory of his own: the dead are lonely! .. a bunch of living people turn up waving stupid gadgets about but the ghost is just happy to see someone so they try to get our attention but it knackers them out so they can only do it in short bursts.
"seeing as they're dead and everything" - lol