The Fourth Doctor audios
The first ever commercially produced Doctor Who audio was the 1976 Argo Records production, Doctor Who and the Pescatons, starring Tom Baker and Elisabeth Sladen as the Doctor and Sarah Jane Smith. To be honest, it is not fantastic. The plot is ripped off by writer Victor Pemberton from his Second Doctor series Fury from the Deep; there are only three actors (someone playing the villainous leader of the alien Pescatons), and so Baker switches from dialogue to linking narration rather jerkily; too much plot happens off-stage, as it were; and the solution, as he kills off the aliens and destroys their planet, is rather un-Doctor-ish. One for completists like me, but most of you can skip it.

Even more so the much shorter Exploration Earth, also from 1976, an educational radio programme which just has the Doctor and Sarah travelling through time and observing the gradual geological development of the planet Earth, with a chrome alien attempting to disrupt things, who is despatched rather casually at the end.