Files synced with BTSync not visible to all apps I have a brand new, rooted Moto G with stock Android 4.4.2.
Being new to Android, I was looking for a painless way to push music to my phone from various computers. I opted for using BTSync, pointing it to my phone's Music folder (that is /storage/emulated/0/Music).

Now, when I put stuff into a synced folder on my Macbook or my Linux laptop, it automatically gets synced to the phone's Music folder by BTSync. Works like a charm.

The issue is that certain apps on my phone do not 'see' these synced files.

For example, OI File Manager, AntTek Explorer and VLC see them and I can play back all the audio files with VLC.

But Play Music (from Google) or Pimp My Music just show me an empty playlist. Also, when I connect the phone via USB cable and use Android File Transfer on my Macbook, the Music folder is empty, even though the aforementioned file managers, VLC and BTSync show the files as existing and, as I said, VLC can even play them!

What's more, when I launch Play Music directly, e.g. from within OI File Manager, it will play back the resp. selected audio file too.

What am I missing here?
The missing piece of information is that Play Music (and most other music apps) don't go round looking for music files. It would be a waste of battery if every app had to scan the storage for files! Instead, there's one built-in app, the media-scanner, that looks for music files and makes a database of their metadata: that is, the app reads the ID3 tags from each file and stores the track name, artist, album and so on. Any app on the device can access this database to find music files. (It works the same way for videos and pictures.)

The app has to know to update this database, so when an app downloads new files, it has to tell this media scanner the new files are there. Obviously BTSync isn't doing this.

On older Android versions it used to be possible for an app to just tell the media scanner to scan everything. This was a useful workaround for when apps like BTSync didn't do their duty, but it also meant some lazy app authors did this instead of telling the media scanner what files to scan, wasting the battery. For this reason, it's not possible to do this on Android 4.4.

In the first instance, you should ask BTSync's author to tell the media scanner when it downloads a file. Then there won't be any problem at all. Until they fix their app, you should try one of the workarounds in this question.