One (1) file extension per folder, since all of the files will have the same filename (the book name). So, 7zip the audio files, and drop the single 7zip file into the Calibre folder. Can't use .zip because that is an e-book format.
If you have too many to deal with manually, or you have pictures or other media files or images, see:
https://www.mobileread.com/forums/sh...d.php?t=277104. It allows you to set all types of custom column values and Identifiers and all of the standard metadata values automatically based on dynamic variables specific to each "book". Also, it can create an Identifier that has the original path of each "book" so that is never lost, even if you delete a custom column.
DaltonST