In this case, I bet the R is the clue. The author probably has some sort of non-rhotic accent where the vowels sound much more rounded. The R would give a bit of the brassy trill sound, and they would probably add the M to color the sound of the vowel so that the word has that bassy ending. I'm guessing that if they tried reading BWAAAA it would sound too rounded and lyrical, and BRAAA would start to capture the early timbre of the sound, but would still probably end of a higher, less aggressive note.
I can't really imagine exactly what accent causes his BRAAM to sound like our BWAAA, but you can kind of imagine what I'm talking about by imitating a guy from Boston saying BWAAA vs. BRAAAM.
Edit: It appears the author went to Swarthmore. My brother went there, and the general consensus is that it should be pronounced non-rhotically. To my ears, the way they say the first phoneme sounds like "SwaAH"[1] (like a person who is surprised, but also non-plussed), whereas I would say it like "SwaRRR", in kind of a gutteral/piraty way. So I wouldn't be surprised if he thought BRAAM approached the gutteral/brassy sound of the effect much better than BWAAA.
[1] I don't know IPA. If any linguists are reading this, I'm sorry. Please feel free to correct anything I've said.
The "R" is presumably because these sound also tend to have a rolling sound to them (reverb?). Think brrr or purr.
You could probably say something like PREEEB with similar effect though.
The ending has a harsher tone to it compared to the bassy open sound at the beginning. I think it's an artifact from the reverb maybe. I know analyzing it in text is difficult but I hear it like the beginning is BRAH. then the fade out it AAMMMMMmmm.
But people remember Jaws, Terminator, and Aliens.
At some point somebody is going to make The Searchers of robots beating everyone up
It's about time we got something like that: https://xkcd.com/311/
There was that 1st person movie, which actually was kinda cool, but it made me nauseous.
As far as the origin of the sound itself, seems pretty obvious that people are just oversampling a Klaxon.
Klaxons were designed to make a very specific impression, so even if they aren't oversampling a Klaxon, I am not surprised if they are subconsciously arriving at the same result. But I'm pretty sure you are on to something -- I recall a monster sound being specifically based on a klaxon. I believe it was either the War of the Worlds tripod, or the Cloverfield monster.
http://half-life.wikia.com/wiki/Developer_commentary
Never knew about this until reading your comment.
While it's possible this is true, I think it's more likely that this is just survivor bias at work. Nobody remembers the terrible summer blockbusters from years past. This years robots destroying cities were the Westerns and Noir of years past. Everybody remembers Casablanca, and The Good, The Bad, and The Ugly, and these titanic movies overshadow the Noir and the Westerns that they were deconstructing. At some point somebody is going to make The Searchers of robots beating everyone up, and another genre will step in to the role of mass-market crap.
I think the actual argument in the case of the "BRAAM"[1] sound is that Inception is a genre defining movie, on the level of Jaws or Il Brutti. It's become a shorthand for Earth-shattering collisions, in the same way Sergio Leone's soundtracks are synonymous with the west, and the Jaws theme is synonymous with being stalked. You already hear people imitating it to give additional context to stories.
[1] Sidenote, because it bothers me: The sound seems much closer to "BWAAAAA" than "BRAAAAM". This is not important, but it bothers me.