How to get rid of seasonal mildew smell?

I live in a typical 1100 sq ft ranch house on a slab with the long hall connecting the living room/kitchen area to the bedrooms. There is a built-in cabinet in the hall that smells like mildew in the winter. The inside of the vanity in the bathroom that backs up to the same space from the other side also smells like mildew in the winter. It goes away for the most part in the summer. Should I be concerned? Can I do anything to fix it short of opening up the walls? Thanks!

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