Over and above that, let's assume that EVP supporters have the best case possible.
1)The sounds were not recorded from the room, or faked.
2)They were not radio signals.
3) They are clear, unmistakable voices with clear messages.
Giving them every benefit of the doubt, this proves one thing, a voice appeared on the tape through unknown means. To jump from that fact to the existence of ghosts or spirits, isn't rational, there's no evidence to make that leap even if you clearly have a recording that can't be explained by conventional means.
As much evidence that there is that such recordings are "ghosts" you could equally say that they are aliens, or the voice of GOD, or that broadcasting devices from the future send signals back in time. All of these are extraordinary claims requiring extraordinary evidence.
The best you could say of EVPs, if all were conducted with honesty and conventional explanations were all ruled out, is that voices appeared on a tape for unknown reasons.