Wondrous item, common
You can apply this wax to any armor that imposes Disadvantage on Dexterity (Stealth) checks to negate this effect. This negation is not a source of Advantage, you can benefit from Advantage on the check as if not wearing armor.
The wax takes one hour to apply to armor and consumes the whole block. You can do it yourself during a short or long rest, but it can also be done by another, for example a squire or other attendant from the Noble background. It lasts indefinitely, but each time you take a Dash action, make a Strength (Athletics) check, or spend a day traveling in the armor, roll a d20. On a 1, enough wax has worn off to lose this effect and the armor again imposes Disadvantage on Dexterity (Stealth) checks.
Originally I was going to do an earring or something with a similar effect, like the one worn by Jarlaxle in the Drizzt books, but then I had the image of a knight professing he had a magical item that quieted his armor that was really just a bunch of linen stuffed between the plates. That evolved into this. As a consumable, I would charge 50 gp for it in normal conditions.