After a couple years of use my bike horn stopped making any noise when air flowed through. Even after a fresh air tank refill to 80 psi, it would only whine quietly for a second before hissing to depletion.
I confirmed that the volume knob was not limiting airflow,
and that there were no kinks in the hose beneath the air tank's bottle cap cover,
so I turned to the faq:
If it's still 'windy' try turning back cap to the left until it pops off. Then check to make sure membrane is flat & clean (spiders have been known to move in). Reassemble, then push & turn back cap all the way back on.
Sure enough, that round piece on the back of the horn pops off after a slight turn to the left:
I didn't see any spiders inside,
but I ran water through the tube to clean out any dirt—
—and that did the trick. It's now loud as new.