This will play an air horn when you have aggro on any of the bosses in blackwing lair. It will only work if you have the target selected.
Delete individual bosses as needed.
MC Boss & Onyxia Raid Aggro --> https://wago.io/S2jur1PY-
Gouryella Raid Combat Rogue [Classic] --> https://wago.io/cIPQ22y3F