Last show of The Mercury Theatre on the Air series, and it's lost, so we don't get to hear Welles speaking about the impending transition to The Campbell Playhouse, in addition to not being able to hear how he tackled Wilder's book.
Wiki:
(The Bridge at San Luis Ray) tells the story of several interrelated people who die in the collapse of an Inca rope bridge in Peru, and the events that lead up to their being on the bridge. A friar who has witnessed the tragic accident then goes about inquiring into the lives of the victims, seeking some sort of cosmic answer to the question of why each had to die. The novel won the Pulitzer Prize in 1928.
From THE MEDIUM AND THE MAGICIAN by Paul Heyer:
(The broadcast of) Thornton Wilder’s The Bridge of San Luis Rey on December 4th featured some spectacular auditory conjuring for the climactic destruction of the bridge. Welles worked for hours with sound effects man Bill Brown. The collapsing bridge was done with an inner tube slapping vines and leaves against a wall, to which Welles added a sequence of one actor’s scream segueing into another’s. It did not come off exactly as he wanted but still sounds effective. This broadcast would be significant as the last one The Mercury Theater aired under its own name.