The sound is almost certainly that of a human voice (source unknown). It’s resonates from about 923Hz to 1410Hz. Unfortunately, there is a quite a bit of noise in this range making complete isolation very difficult. In the clip I am providing you, the low-range Buh and Fuh sounds from the beginning of what seems to be the world Be and Fine have been isolated out and therefore causes this version to sound more like You’ll-hee-hine. The result is however, a fairly clean version of the audio anomaly.

You’re tape player appeared to be slightly fast in either recording or playback, as I tracked the ambient EMF, which is very audible at 58.23Hz. As you know, standard electrical current on the power grid resonates at 60.00Hz almost perfectly. Transmission companies ensure this to prevent problems between different power plants generating onto the same grid. So this can be used as a fairly reliable measure in processing to identify the correct playback speed.

When comparing the detectable reverberation between the voices heard at the beginning of the tape and at the point of the anomaly, we see a minor difference. Now, you have indicated that this in an enclosed space and I understand the door was open when they were conversing and it was closed at the time of the anomaly. I also understand it was a sound-proof enclosure, but I do not believe as you have not indicated that it was anything like an anechoic chamber. The reverberation of the anomalous voice is definitely there, and amplified. The reverberation length (on average) in the beginning of the clip (with the known human voices talking) is about 152ms. Where-as the whisper appears to reverberate detectably for about 185ms, which suggests to me that the sound was in fact audible; the microphone reacted to sound waves, and then reacted to same-signature waves on a delay.

The audio of the voice does not seem in any way super-imposed onto the recording. I am almost 100% sure that if a human being were sitting in that enclosure, they would have heard it.