Florian and Ohrenkino, thank you so much! Problem solved.
One last question, admittedly of little importance IRL: When the column header is Last Modified, what is the string operator?
I understand now that the problem was about correspondence between column headers and operators in the format string. And about the underscore for column headers Path and Filename.
It is very convenient to use %_filename_ext% when the text file and the music files are in the same folder.
Sometimes they are not. For example, music files which I want to batch edit can be in the same folder but divided into subfolders. If I have got it right this time, the operators %_path%;%_filename% would be appropriate.