This is an onomatopoeia used to convey the manner in which a concert band should not begin a piece of music.
This sound is typically produced when the musician(s) is not prepared to play, did not breathe in an effective fashion, or is not using air that is fast enough to develop an acceptable tone for his/her instrument.
Band Director: "I don't want to hear the trumpets make an entry like that again. Articulate and be ready to play. I want to hear a clean 'ta' not a 'bmrahw.'"