Thanks for the heads up but I don't have access to the Field Guide. Here's where I'm up to with my own experiments...
This Hazel rule watches the Smart Folder for any new tagged files/folders:
This triggers a KM macro that recursively checks for any .m4a files and copies them to the Dropbox folder if found.
TEST.kmmacros (22 KB)
If it is possible to delete files from the Dropbox folder when their parent directories become untagged, it won't be simple, so I might quit while I'm ahead!