I think I've finally found the most comprehensive list of Robert Montgomery appearances on radio, thanks to RadioGoldIndex.com. Count 'em, 71 programs!
Not too long ago I was flipping around XM Radio stations in my car and popped on XM Radio Classics. An episode of Suspense was ending and Bob's voice seemingly came out of nowhere as he added the show's end narration. Odd how you "tune" into things like that.