Outside of adaptations of "the 39 Steps," the only other old-time radio program adapted from Buchan is his short story "the Grove of Ashtaroth," as heard February 29, 1948 on Escape. This tells the tale of a man who builds an elaborate home in Africa by a grove of trees. A visiting friend soon discovers terrible things transpire in the grove by night. Buchan's biographers have pointed to this tale being inspired by Buchan's adulation and later disappointment in T.E. Lawrence.
You can download a copy of this radio show from archive.org here.