Well, it didn't work. It now seems that because I created the folder myself, TB no longer has permission to "open" the file as of version 12.0. I don't know why. I've tried everything I can think of, but nothing works. I copied the .wav file to the downloads folder in my home directory, & it even works there. So, it's my folder. I've allowed read & write access, but it still doesn't work. Perhaps TB has now "locked itself out" in some way? If anyone can figure a way to make this work, I'd greatly appreciate it!
In that case I would suggest backing up data from T/bird, uninstall it completely, and ensure all relevant files get deleted, reinstall it and bring back in the data.