Kestrel,
Stepping lightly around an ethical debate, eMule is pretty effective file sharing program for finding good examples of such programs. Not that I would know anything about that...
If that doesn't mesh well with you, there are free streaming old-time radio programs online like live365.com. Such programs would keep playing continuously through the night, though, so you'd have to keep it at a lower volume probably. I believe there are other such websites, if you try Google.
Lastly, they are widely available in bookstores in small collections, with just 'Suspense' programs, mysteries, 'The Shadow', 'The Green Lantern', 'The Mysterious Traveler', etc.
Personal fav's are The Shadow, Mysterious Traveler and Sherlock Holmes.
Sweet dreams folks!
-Spooky, who can tell it's going to be another late night