Using Explorer, right click on the parent folder of your MP3s and say "SEARCH"

Search for *.* or *.MP3 whichever you like.

The result will be a sortable list of every MP3 in and below that folder.

You can either work with that list directly, or press CTRL-A (select all) and either move or copy the entire file list to a new folder where they will all be "flat" instead of in subdirectories.