A recent thread here lead me to The Internet Archive, Old Time Radio section, where you can download free a ton of great audio: Link

X Minus One (and its ancestor Dimension X) was a sci-fi radio program on NBC in cooperation with the Astounding and Galaxy pulp magazines, which ran many classic stories by all the great sci-fi writers of the day.  They also ran a ton of crap.  X Minus One made radio plays of both the classics and the crap.  I listened to all 125 episodes at work and I've filtered out some of the better episodes, in my opinion.  Some are lighthearted, some are prophetic, some will get you right in the feels.  I'm linking to Youtube copies because I can't figure out how to link directly to Archive episodes.  You can go straight to the Archive and listen to them all, and many others: Link

Heinlein - Universe
Asimov - Nightfall
Bradbury - The Veldt
Robert Sheckley - Skulking Permit
Eric Flint - A Pail of Air
Gordon R. Dickson - Lulungomeena
Bradbury - There Will Come Soft Rains
(Bonus - ultra-creepy Soviet animated version of There Will Come Soft Rains)
Robert Sheckley - The Native Problem
Bradbury - Mars Is Heaven
Tom Godwin - The Cold Equations
Murray Leinster - A Logic Named Joe
L. Sprague de Camp - A Gun For Dinosaur
J.T. MacIntosh - Hallucination Orbit (audio is a little muddled, I recommend you go to the archive, crisp audio there compliments the great voice acting)
Murray Leinster - If You Was A Moklin
PK Dick - Colony
Asimov - Hostess
Michael Shaara - Soldier Boy
Heinlein - The Green Hills Of Earth
Skipping to what is IMO the best moment of X Minus One - The Green Hills Of Earth - "Bone tired the race, that raised the towers..."  Perfectly captures the 50's ancient Mars

Couldn't find Frank Robinson's The Reluctant Heroes on Youtube, I recommend you go to the Archive link and check that out as well.

One of my favorite series. I have every episode on an MP3 CD that I listen to on road trips.

Excellent choices.

Good times.

I love those old radio mystery, horror and SciFi radio programs!

You can actually find a few iTunes apps that have the radio programs.  I listen to them on the road.  The horror ones are great for a late night drive.