Syncing playlists between Ubuntu and Android smartphone
Spotify allows you to sync your local files to a mobile device (no subscription required). Of course this requires the desktop client installed on a computer and the app installed on a mobile device. Once your local files are downloaded to the device, you can listen to them whenever and wherever you want. It also does not require a data connection (after downloading). 

*Note: the later versions of Android client have had issues with this. Give it a shot anyway! The only workarounds are to install an older version of the Spotify app and then download your playlist.