These are the audio recordings from the celebrated Feynman Lectures on Physics. These recordings were released as a series of twenty audiobooks, each of which contained five to eight (assorted non-consecutive) lectures. The posted links collect mp3s of these lectures in consecutive order. The collection is nearly complete, but it is missing the recordings from the following lectures:
Volume 1, lecture 23
Volume 2, lectures 28 and 29
A typical Amazon product description for the audiobooks (the one for the final audiobook, ISBN 0738208787) is pasted below.

Series editor David Pines has selected, from the more than one hundred recorded lectures, the six that address the greatest physics discoveries of the past five hundred years. In these lectures, Feynman not only explains gravity, relativity, probability, electromagnetism, quantum mechanics, and superconductivity, he offers his own unique take on what made these discoveries possible. This is a wonderful opportunity to hear Feynman expound on the contributions that have led to our present understanding of the nature of the universe. Volume 20 (The Very Best Lectures) is the concluding volume in the series-and an extraordinarily special one.