Oh god, not this baloney again. Seriously, you'd think that living in an enlightened age such as this... Anyway.
Short short short version: Recording equipment with bad radio frequency shielding + Psychological phenomenon that makes us recognize language.
Unreliable version:
http://en.wikipedia.org/wiki/Electro...m_explanations
Longer version:
http://www.randi.org/jr/042205modern.html#11
Even longer version:
http://skepdic.com/evp.html
Of course, a lot of people will chose to believe they're receiving message from the great beyond, not from the baby next door or someone's walkie-talkie. Fun times for them.
For the rest of us, there's Google.