Besides the obvious answer of extinct animals. My first priority would probably be to record ambiences without the presence of human civilization. No cars and traffic, no aircraft, no electrical interference from cellular and radio...just pristine nature.
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bird life in New Zealand before any humans arrived.... esp the extinct Moa and Huia
My children growing up......I have a few bit and pieces, but I now wish I'd recored them more regularly, say once a month. Clique, but they grow up so fast and before you know it you have missed key bits of their development.