On your Clipboard: OPTIONS > PREFERENCES > Game Sounds. Find the "stall horn" sound in the list and tune down the relative volume.
I have customs souds (Waffle) and settings as follow:
Internal engine (normal eng, Jet, vehicle and boat): 50%
Flaps: 100% (the custom sounds are pretty low in some planes)
External eng: 100% (to hear enemy planes appproaching)
Stall buffet: 6% (thats the annoying machine gun sounds that make you think your being shot)
Debris Hit (when you crash in shot off parts): 50% (those always give me a heart attack