Hi Tim,
Not that it helps a lot, but I’m not having any issues playing audio-files (.mp3). I take the same steps you do:
So, I guess the steps you follow are correct.
I do use the filemanager widget to upload the audio-files, instead of the dropzone. I guess you made sure the Name attribute of your FileDocument was set correctly after the upload, the object was committed, and the file has contents?
If the file is synchronised to your device, you should be able to find it by name in a nanoflow.