To be honest, I have never in my stint in rails heard that alarm before, and that kind of scares me. I know its NOT from power being killed (much higher pitched) and its not a MAPO alert (they wouldnt be that loud, and sound much different.)
All I can think of is its maybe some sort of fire alarm someone pulled to attract emergency services to the site.
Edit to add: That video really shakes me up, not just because the fact that someone has just died there. But because, that was my workplace. We always ran such a tight ship there. In that video, SO MANY things are happening that are totally foreign to us. Apart from the obvious, two trains smashed together, guests are scrambling out of doors opened under Emergency circumstances, loud sirens are blaring that, most likely, none of us have heard before; gates are flung open and Guests are crawling up on them. Monorails is, obviously, not supposed to be like that, and I hate seeing my coworkers (I know both people in that video) being thrust into such a foreign situation. I can't really explain it farther, other than it is deeply disturbing to me, as a Monorail CM, to watch and listen to.