Thank you, Old Toad.
I was able to track down all the .m4a files in the Finder. Unfortunately the file names (“Call with so-and-so” or “voicemail-”). and their creation/modification dates and times are not the dates/times when they were created on the iPhone; they are the dates/times when the Note containing the .m4a file was modified on the iMac.
The file that I originally posted about has the correct suffix but the wrong (red Music) icon so that I can’t play the recorded conversation from inside its container Note as I can other Recorded Calls by clicking the blue triangle icon. Back in the good old 1990s, we had an Apple utility, ResEdit, for fixing this type of error. I still have the book and the installer diskette (1991). If only . . .