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,

Back side of Airzound volume knob

and that there were no kinks in the hose beneath the air tank's bottle cap cover,

Airzound bottle cap

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:

Opening Airzound

I didn't see any spiders inside,

Airzound internals

but I ran water through the tube to clean out any dirtβ€”

Rinsing Airzound with water

β€”and that did the trick. It's now loud as new.