Change music tracks by volume buttons.

You need a media player app to play tracks. This app enables you use volume button to change current song.

Press volume button ONCE for changing current song.
Press volume button TWICE or more in quick succession to change the volume.

Some phones kills background processes when screen is off.

So PLEASE disable battery saving feature for this app so app can work as specified. You can check your phone documentation for more info.

Your device will vibrate briefly when app changes song. You can disable vibration from app.