You could try soaking them in warm water for an hour or so, then towel drying, then wearing them around till almost dry... We used to do this in the 60's with handmade imported sandals made in Mexico or India.
I have always used baby powder. It provides immediate squeak relief, plus it keeps your feet smelling fresh!
Dust with talcum powder.
When the doctor gave me my orthodics, they squeeked and I was told to dust them with talcum powder. It did the trick!
Sometimes it just takes wearing them awhile to break them in and get the leather conditioned. Wear them around the house for a bit.
I have heard to use talcum powder for squeaking floor boards. Wonder if you could use that? Being powder it might be easier to get in there easier. Only thing I would worry about is if it would cause the leather to dry out prematurely. I think I would call a cobbler/shoe repair shop and see if they have any tricks to try.
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