The Valentich tape was played to FSO's on course in Henty House - to demonstrate how one should act when one was under the pump like the FSO was .. I was on one of the many courses in '89. I've also subsequently listened to it again since then.
The FSO outwardly, sounded cool calm and collected, the reverse could be said for Valentich.
Angel, yes, to answer your questions - it did sound legit and believable.
Having been on the other end of the maydays, I'd take a lot of convincing that the transmissions were anything else.
Having said that, there are of course, others with converse opinions.
As for the legality of the tapes .. remember that the original recording was on to the tapes that belonged to DCA/CAA/AsA and it's many other variants
, as such, any copy of those tapes would have, shall we say, not necessarily been kosher ... nothing sinister in that.
So Angel and PPL ... chill ...