The IDF originally released the audio, stating that it was an exchange with the Marmara. The activist in question - the only person identified on the tape - was not on the Marmara. This is either a 'an honest, and rather insignificant, mistake' or else an indication of fabrication.
The IDF are now saying that they have no proof that the recording came from the flotilla ship - but this is also very odd, as even on an open channel, they ought not to mix up audio signals from two different ships.
If you listen, the anti-semitic remarks come in suddenly, with gaps which sound very like edits. No one has been able to identify those voices, and for some reason the IDF is not releasing the radio log of the seized ship, which could verify its claims.
But the pro-Pali debunker recognized his own friend’s voice on the tape. The voice of a friend who was definitely on one of the flotilla ships.
Looks to me like the debunker ends up lending quite a bit of credence to the recording.
On a single channel, you cannot tell the origin of a signal. It's like an old fashioned party line. That's why the IDF navy identified themselves when calling on the radio. You can't tell if a different radio came up, or someone took the microphone at the one you'd already been listening too.