Simplify the following text for a Grade 10 native English-speaking person.
Original: William R. Rathvon is the only known eyewitness of both Lincoln 's arrival at Gettysburg and the address itself to have left an audio recording of his recollections .
Simplified: William R. Rathvon is the only known eyewitness of the Gettysburg Address to have left an audio recording of what he remembered .