In my five years of ghost hunting, I've caught many EVP as has my wife. One particularly compelling piece of evidence we sent to an audio engineer who used to be a friend of ours, for strict analysis. This guy didn't believe in ghosts and, yadda-yadda so we figured he'd tell us straight what we had. Bare in mind this was one piece of many we've collected over the years.
That night the guy called us with a disgusted-but-excited, "How'd you do this," voice. According to him the audio, the 100% unedited version we sent to him, was 30% faster at the EVP anomaly. In other words the voicing was fine before and after the anomaly but, supposedly, during the spread of the anomaly the voice that came across was 30% faster than the ambient background noise. He stated that this was "impossible". I would say it may be analog-bleed but this was digital media.....taken on the Zoom H2 in non-normalized .wav files. I wouldn't say this turned the guy into a believer but he was certainly perplexed. Unfortunately we ended the friendship before he had the chance to review any more audio so we're not sure if the results are replicable.
David Rountree believes that EVP is some type of electronic interaction between the entity (ghost or otherwise) and the actual audio electronics being employed. He's got some pretty technical explanations about certain types of mic's and receivers in his books supporting his hypothesis. David's explanation argues that the phenomenon occurs inside the device. Other people believe it's a type of frequency manipulation detectable by electronic microphones but not the human ear. This explanation argues that the phenomenon occurs outside the device.
Some less-knowledgeable ghost hunters believe that things such as white noise and cheaper audio recorders give better EVP. I say such things just yield more false positives. If EVP is real then it's a phenomenon that will improve with sound quality. The better, more clear of audio you have, the better the EVP, and higher quality equipment and methods will produce more genuine EVP, not whispery, ghostly, "I don't know what it is but I think it's a ghost!" audio. Of the hundreds of EVP files we have most are just that, whispery, thumpy, elusive pieces of audio but there are a few high-caliber hits in there. I do believe in EVP, but understand some people's reservations about it.
My 2 pennies.