When I started half-listening to her show on WAMU a couple years ago, I didn’t know Diane Rehm was having voice problems, even though the shaky croon of what should have been a really, really old woman I heard sounded problematic all over. But the people in charge of the station didn’t seem to think so. And the words she spoke during her programs were strangely soothing. Listening to her became a substitute for watching television news reports and reading the paper, at least until other people began sitting in for Rehm on what was, after all, her show. Only a mysterious voice like hers could hold my attention. She explains what happened and more in her autobiography, Finding My Voice, and in a conversation with Nightline host Ted Koppel tonight at 7 p.m. at the National Press Club Ballroom, 529 14th St. NW. $5. For reservations call (202) 347-3686. (Ayesha Morris)