The reed in the stall warning horns of 150/152/172's is in the horn itself, which it just behind the plastic trim, right at the corner of the windshield. The noise cancelling headset will reduce, but not eliminate the sound of the horn (the headset people thought of that). In any case, when you're flying in a phase of flight where a stall is more likely, you should be aware, and attuned to the possibility of a stall warning. It's pretty common for a 172 to be flown around, and the stall horn never sounding, but it'll be a little better flying if for some landings it does. It you have the stall horn peep just before the mainwheels touch, it's set up to be a good landing, at the slowest practical speed. If you doubt the stall warning is working, at altitude, and clear of traffic, bring the plane to the approach to stall, and listen for it! The reed horns do need to be blown out (bugs) from time to time. If the plane has had major repair work, or a wing adjustment, the stall warning should have been checked, and adjusted if needed (my job, following STOL kit installations, in a former life). Recently, while test flying a Grand Caravan, I was getting the stall warning, just as the mainwheels touched, and they were satisfying landings....