The Rav gave a famous shiur at a Pidyon Haben in 1974. During the shiur, he descibed his experience as he gave his Talmud shiur in YU as an old man. That part of the shiur is around 9 minutes long. The quality is pretty clear. In order to make the tape easier to follow, the tape was transcribed word for word. (This speech has been reproduced twice already, but neither one is literally word for word.) This transcription has been uploaded as well. You can follow along as you listen. The tape is very powerful and inspirational.