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.