A ".M3U" file is not actually an audio file or a codec, it merely contains a playlist of audio files that can be read by a media player, copying or moving "M3U" files won't copy the audio files listed within.
You can actually see the list of files by opening the M3U in Notepad or by dragging and dropping it into a Notepad window.
What you need to do is go back to your media player, load the M3U file there and then find the location/path of each file contained within and have Switch convert each of them or all of them in a list.