I use BTSync to push music to my Android phone. Music player apps do not 'see' these files because media files are not found by scanning the file system (too much battery) but by looking up a database that is updated by the media scanner. Because BTSync apparently doesn't seem to run the media scanner after syncing files to the phone, these music files never show in any of the commom media players on Android (except VLC, which doesn't use the database but does scan the file system ... but I don't really want to use VLC to play back my music). See also this question on Stack Exchange for Android. Aka: this is a feature request.