I've heard of this happening before as well. I've lurked far more than posted [by the time I catch up on a little reading I usually don't have time to post lol], I recall a member here posting about a voice coming through the computer which responded to her questions if I recall right [believe it was Anvil, sorry if I have the wrong person], but had heard of it happening before then as well.
If DRV communication is possible when not the computer? I don't know that it's usually the case [discarnate voices] though, I suspect that more often then not the source has been/is mundane [as others have explained has happened to them], but that's just speculation [on my part].
When it comes to the PN the question is not only what do we know, but, what don't we know? Difficult to draw lines in the sand, that is to say what is and is not possible, at this stage of the game, IMO.
Research that various scientists [and others] have done on EVP/DRV [ITC] over the years has really been quite remarkable. The work of Dr Raudive in Germany, Friedrich Jurgenson in Sweden, Peter Bander in England, Marcello Bacci in Grosseto, Italy, or Professor Walter, Alexander McRae, Professor David Fontana, Dr. Anabella Cardoso, & others such as George Meek, the Butlers, just to name just a few.
I fail to see how Media Skeptics [well I use the word skeptic loosley, IMO psuedo-skeptics, cynics, scoffers, debunkers, is more accurate] such as CSICOP, Randi, et al, can outright dismiss it [as some have] given the results obtained, in particular with controlled experiments done in labs and sound proof studios with screening etc.
I'm not aware of any one explaining these classic cases on a scientific basis [that is in a credible scholarly rebuttal after critical examination], actually with some of the dismissals I've read this research wasn't even touched on, nor was any evidence provided [based on extensive research done] to support their claims [and dismissals].
During some of the experiments other skeptical minded scientists attended and referred, they were unable to explain what occurred [same for the electronics/sound experts on hand].