Unsatisfying, Hanlon-style prediction: no one involved had any notable malice or competence.
Maintenance and cleaning crews love to listen to radio/music on speakers while working. Someone from this staff plugged in a receiver to listen while servicing the plane, and forgot it. Or they left it there for convenience and didn't think anyone would notice. Someone else unrelated, while on the flight, found this receiver on their phone/laptop, and played these sounds, possibly unwittingly.
AA may have found these devices after the incidents and just not reported it. Or they didn't take the reports seriously enough to comb the plane for problems. Or they gave up and would rather not lose the money by grounding the plane and tearing it down. (I don't have any aviation expertise, so I don't know exactly how disruptive it would be for them to to do this.)
This seems like the most likely explanation. Whoever left the device in the plane isn't likely to own up to it (if they even remember where they left it!) since they may lose their job.
Maintenance and cleaning crews love to listen to radio/music on speakers while working. Someone from this staff plugged in a receiver to listen while servicing the plane, and forgot it. Or they left it there for convenience and didn't think anyone would notice. Someone else unrelated, while on the flight, found this receiver on their phone/laptop, and played these sounds, possibly unwittingly.
AA may have found these devices after the incidents and just not reported it. Or they didn't take the reports seriously enough to comb the plane for problems. Or they gave up and would rather not lose the money by grounding the plane and tearing it down. (I don't have any aviation expertise, so I don't know exactly how disruptive it would be for them to to do this.)