OK, I never looked into this at all and didn't go to the site you listed. But I can tell you that there are a few ways that "voices" and other sounds could be caught on tape recorders. Allow me to use bethel, of all places, as a prime example...
Back in 2000 when we went on a tour in Brooklyn, one section we passed by was the tape recording/packaging area. The room where they actually recorded audio to tape was built into a Faraday-like cage. Our tour guide didn't know about the Faraday cage, so I explained to her that it was designed to restrict electromagnetic fields from entering the recording equipment. You see, we are constantly bombarded by radio waves no matter where we go. Some electronics equipment, if not properly shielded, can introduce some of these electromagnetic "harmonics," if you will, into their output stages. Years ago, most transmitter equipment utilized analog output stages that could be eavsdropped on with a matched frequency receiver. These days, most of the airwaves are dominated with digital streams that cannot be eavsdropped on with ordinary analog equipment. But these sources still exist.
It is highly likely that EVP is simply a case of taking a poorly and/or unshielded analog input and "accidentally" introducing an analog "over-the-air" electromagnetic wave to its circuitry. Some of this equipment may pick up bits of a cordless phone conversation from down the street and others may not. It's really hit or miss.
So do I believe it to be true? No.