This will play an air horn when you have aggro on onyxia or any of the bosses in molten core. It will only work if you have the target selected.
Delete individual bosses as needed.
BWL Boss Raid Aggro --> https://wago.io/M-uPaDDpR
Gouryella Raid Combat Rogue [Classic] --> https://wago.io/cIPQ22y3F