I have a similar kind of fun story. The emergency phone on the elevator at work had a phone number somewhat close to that of an appliance store. People would call the number, the elevator phone would immediately put them on speaker phone. I figured this out one time I heard a faint phone breathing sound, and I started speaking with the caller. This happened again about once a year, and I had a fun time explaining to the callers that not only did they have the wrong number, but they had reached an elevator emergency phone. This response always caused a great deal of confusion.

Okay thank you for sharing - I answered a spam call on an elevator emergency phone once and I was puzzled but found it funny. So there’s an actual number…oh boy if only there was a registry…

Yeah, unfortunately I couldn't get the elevator number from the callers. I really wanted it to play pranks on coworkers.

I've received a spam call in an elevator. The call asked me to press 1, and I'm still bummed it didn't work.

Was there an AM transmission station nearby? Sometimes some metal objects can act as an antenna-speaker combo when there's a high power AM transmitter nearby. (some people have even reported this happening with dental fillings!) I'm assuming this is a bell or some other low-tech thing, if not there are probably many other explanations

This gives me an idea of an attack vector. These groans and moans seem rather quiet, and the airline claims its an amplifier issue. Assuming that the amplifier is not shielded from the cabin and that its analog tech, one could theoretically induce intentional interference to play whatever you want, right?

Not to my knowledge, but it's possible. There certainly wasn't a radio tower around, but there were antennae all over the place, as there are on any large campus. I don't think that's what was happening; it seemed to be different alarms at different times, but I believe it was always the same song. Which seems to me like they were being randomly selected by a script. These were fancy fire alarms and it was 2014, it's entirely plausible in my mind that they were networked and running a vulnerable embedded Linux. I don't think it'd be a Mission Impossible level effort to make this happen.

But, I'm going off of 8 year old memories, so the only thing I can say for certain is that it happened.

Someone in my engineering school opened the elevator panel and had it play a clip from the matrix (the full quote about there being no spoon) when you pressed a floor button.