While discarding more of my overflowing files, I came across the following, from a distant cousin:

“My grandfather was the one who sang hymns in his sleep, but never sang when he was awake. He stayed with my parents quite a bit after his wife died,  so I got to hear him sing while he was sleeping many times.  Some people talk in their sleep, but he would sing. Never heard of anyone else that sang in their sleep.”  (Nor have I!)