[kosciusko51]

It does sound like some sort of hack of the audio system.

If it has happened multiple times, I wonder if they cross-checked the passenger list to see if the same person was on the flights.

[Waverunner]

More likely an employee who's in the avionics section for overhaul and repair.
Full access, knows the system, just be glad he's doing a prank instead of something sinister.
Unless of course it's really the ghost of a white supremacist.