I don't have a long history of owning leather shoes. My shoe budget typically doesn't go higher than $60, even for running shoes (I snag them on 50% off sales, for example). I have a nice pair of leather "walking shoes" that have a squeak, though.
I bought them about a year ago and have worn them sparingly, both due to the fact that I want to keep them nice and because they squeak. I've tried searching and a lot of recommendations talk about the heel. These don't have squeaky heels -- they have a squeaky tongue. It's a matte or non-shiny leather, black, and from what I've read I need to use a leather conditioner.
But as I've never had a pair of shoes that even required polish, I don't know if this is going to do the job. I know a tin is usually cheap, around $5, but I also don't even know what kind to get. I know how TO polish/apply conditioner, but that's about where my knowledge ends.
I know it's the tongue because if my foot is off the ground, I can make the shoe squeak by flexing my foot and the tongue shifts against the other leather.
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Another thing to consider is whether or not your shoes have shanks in them. It's not uncommon for the adhesive that holds them in place in the sole of the shoe to wear off, letting the shank move back and forth a little. Depending on what the shank is made of, it can cause a very annoying squeak. That, unfortunately, doesn't have an easy fix.
-Ancient Dwarfish Proverb