There are a ton of classic audio drama from the 40s-60s, if your tastes allow. My faves are:

• Yours Truly, Johnny Dollar - "The insurance investigator with the action-packed expense account"

• Gunsmoke - "U.S. Marshall Matt Dillon. The first man they look for and the last they want to meet. It's a chancy job. Keeps a man watchful... and a little lonely."

• The Shadow - "Who knows what evil lurks in the hearts of men! The Shadow knows!"

• I was a Communist for the FBI - "The true story of Matt Cevetik who, for nine fantastic years, lived as a Communist for the F.B.I.!"

And if you would prefer something similar but more modern, The Thrilling Adventure Hour is a long-running stage show in L.A. in the style of classic radio drama. The main segments are:

• Sparks Nevada, Marshall on Mars! - "Along with his faithful Martian deputy Croach the Tracker, he rights the outlaw wrongs on Mars."

• Beyond Belief - "Meet Frank and Sadie Doyle, toast of the upper crust, headliners on the society pages, and oh yes... They See Ghosts!"