Norman Lebrecht has an interview show on BBC3 radio where he chats with interesting people in the arts. His most recent show is a good 45 minute conversation with Stephen Sondheim.
Now, here's the hard part. The show aired last Monday and is supposed to be available on the BBC3 website for seven days thereafter…and it is, except that the website is malfunctioning and if you go to the link right now and click, the playback will start about five minutes into the programme. I figured out how to download the whole thing and listen to it from the beginning but it's kind of complicated…so your best bet is that maybe they'll fix the site before the seven days are up. Anyway, if you want to give it a try or hear it from that point on, go to this page and look for The Lebrecht Interview. If nothing else, you'll want to hear Sondheim debunk the much-rumored story that when he met Andrew Lloyd Webber at a party, Webber said to him, "There's not a note of your music that I wish I'd written." Sondheim says no such thing was said.
While you Sondheim fanciers are over in BBC vicinity, you might want to check out a two-part series from a few months ago called Sondheim's Women — an examination of the female characters in his works. This one's a little easier. You should be able to listen to Part One over on this page and Part Two over on this page.
UPDATE added the next morning: The Lebrecht Interview can be downloaded for the next few days from this page. Much simpler than what I went through.