There are a lot of old radio dramas which have fallen into the public domain, and a good chunk of them are available on the Internet Archive. Here’s an arty one I haven’t listened to, but which seems to be chock full of goodness: Columbia Workshop.
It’s a mix of adaptations of classic plays and stories (Alice in Wonderland and Through the Looking Glass, Hamlet) and contemporary ones by new writers. CBS (the Columbia Broadcasting System, remember?) wanted to show that they cared about educating the public and advancing excellence in radio. Composer Bernard Herrman and many notable directors and actors were involved in the series. You can learn all about it over on Wikipedia.