Agreed on that. I'm a huge fan of old radio serials and that's a great way to cut your teeth on learning how to write and produce a radio play - by listening as those shows explored how it was done and became bloody experts at it.
Also, there are repositories of sci-fi dramas and serials from subsequent eras as well. Where I'm from, the CBC has a massive free archive of radio dramas and plays. And if you want to be current and have some cash to buy the downloads, try out Big Finish for an enormous range of sci-fi audio dramas from Doctor Who and a bunch of other stuff.
Mark