IMO, the "voices" Lou is hearing is simply an artifact of how his recorder works. Notice that he is pretty specific about the model that he requires.
Apparently, the Panasonic RR-DR60 uses CELP (Code Excited Linear Prediction). CELP is a fairly common standard for speech coding, used by e.g. GSM (and probably other digital cell-phone standards), Speex (the speech codec), and other systems that need to store digital speech efficiently.
While I don't know all the details of how CELP works, it basically does something like this: When the audio is recorded, the input signal is divided into short segments (frames). For each frame, the encoder tries to recreate the sound wave of the frame using a synthesizer. The encoder outputs the set of synthesizer parameters that produce a synthesized sound that best matches the original sound. These synthesizer parameters are what is stored in the memory of the digital recorder. When the recorded speech is played back, the stored synthesizer parameters are used to synthesize sound, which hopefully will be resonably similar to the input.
The advantage of this method is that it makes it possible to reproduce intelligible speech at a very low bitrate. Also, since the coder usually is tuned to human speech, it can actually make the speech come out clearer and filter out background noise. That's why digital cell phones often can work resonably well in noisy environments.
One downside is that while the output might be intelligible, it can sound synthetic, and the voice might sound different from the original. Listen to the clips on the page CACTUSJACK linked. Lou's voice sounds almost like a synthesized voice (which it kind of is).
Also, if you try to record something other than human speech, it will likely be horribly mangled. The encoder will try to find the best match possible, and since it's only equiped to reliably reproduce human speech it's pretty likely that the output will sound a lot like human speech, no matter what the input sounds like.
Googling RR-DR60 and EVP suggests that that specific model is the tool of choice for EVPers. Probably because of it's ability to turn any noise into something vaguely human sounding.