In order to be able to access your radio by means of remote control, you do not have to buy any supplementary device: your smartphone suffices. With the Volkswagen Radio Remote Control app your smartphone becomes a genuine remote control based on Bluetooth with which you achieve almost all functions for which otherwise you would press the radio keys.
You cannot regulate only the sound volume, but change for example also from the radio reception to the CD player and navigate comfortably in title lists and radio stations.