On many, many subjects, I think the http://www.desiringgod.org website is just amazing - there's not only great reading, but a wealth of audio available as well.
I also admire Abert Mohler :
For instance:
03-13-2004 Eye Covenant Conference at Boyce College
"The Seduction of Pornography and the Integrity
of Christian Marriage" [Transcript: pdf | doc ] RealAudio | MP3