Q: I want to find duplicate files..

While deleting from a search is perhaps not the best way to remove duplciate files as it doesnt provide a lot of obvious info about where the files came from, you should be able to perform the same search in the Finder using the search bar at the top of any Finder window, and then selecting to search "This Mac" at the top of the window.

I know in linux you can use fdupes from the command line.

fdupes directoryname

I think it's an add-on.  You might check and see if there's an equivalent for mac.

The files in OS X should be indexed by Spotlight, so the search results should be relatively instantaneous. Try going to the Spotlight system preferences and then drag your hard drive to the Privacy list, followed by removing it from this list. This will force a recreation of the Spotlight index, and may help the situation.

If you don't have a lot of .m4a files or if you really want to do this you might want to try searching for all .m4a files.

Hey you can just take help from any app available on mac app store just download apps who support all type of duplicate files scanning including music and other files format. you can download Duplicate files remover  and scan your mac

You can search all m4a files in your mac.If there is numerous m4 files in your Mac, you can try some free duplicates programs, like DuplicateFinder3, the free version is able to detect the duplicates, then you can open it in finder, then delete them.