If an external audio source (e.g. an MP3 player or iPod®) is connected to the AUX input then the audio source that is connected can have a different volume than the audio system's internal volume (e.g. radio). Correct this by adjusting the volume of the input: In the normal view for the AUX source, press OK/MENU, select and then volume setting Standard or Boost.
If the external audio source's volume is too high or too low, the quality of the sound may deteriorate. The audio quality may also be impaired if the player is charged while the audio and media system is in AUX mode. In which case, avoid charging the player via the 12 V socket.