Full face Oxy masks usually are clear but a bit hollow and nasal in tone, not muffled. In a hijack situation the interloper is listening in and that should involve a non standard speaker or mic set up that could muffle things somewhat.
I agree with D.S.'s critique of the Ramjet555 theory.
The pilots are reasonable suspects, no matter how harsh, just as when someone is murdered in a home the spouse is normally on the suspect list.