The Leo Strauss Center at the University of Chicago maintains a digital archive of transcripts and audio files of some of the seminars Leo Strauss taught at the University of Chicago and elsewhere. The full archive may be accessed here.
A few examples:
PDF of seminar on Plato’s Republic (1957) is available here.
PDF of seminar on Rousseau (1962) is available here.
PDF of seminar on Montesquieu (1965) is available here.