Onomatopoeia for the sound created by slam metal vocalists. Presumably, the vocalist making the sound is enunciating lyrics, but they are almost never legible, even if you're reading official lyrics published by the band.
The sound is sometimes achieved through the use of pitch shifting technology.
"Hmm, so Propitious Vegetation's lyrics are about veganism, eh? Well, who knows if that's true, all I can understand is bree bree bree..."
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