There's a problem with #3, highlighted. If you hear something in the recording, you may take that as proof of spirits talking, but it really doesn't do anything to discount the mundane pareidolia explanation.
Is there a second person (scrappy?) who can to make out the same voices as you? If so, we can take one step closer to a protocol.
Another possibility would be can what you hear be responsive to some question? It would need to be a question to which you do not already know the answer, but can be easily verified later. If so, we can take two big steps and a small leap closer to a protocol.
If I hear something within the recording here on my machine, and it alters in the same way on the recording of the participants machine, this indicates something afoot, yes, no?