While it is plausible that the hack compromises the entire comm system, it's unlikely. My bet is that the steward made an incorrect association between the sounds and normal system behavior. These inaccurate retellings are extremely common in ghost stories.
Only time will tell, but I'm almost certain I'm right on this one.
This makes a lot of sense, but I’ll add that it’s also possible that the association was correct, but not for the assumed reason. For example, maybe the plane had a partially faulty breaker for the IFE, and there was a likelihood of tripping every time the PA system was used. (The button probably isn’t a breaker, but there’s a world of similar explanations.)
Only time will tell, but I'm almost certain I'm right on this one.