Question to Xceed driver with standard audio system (without JBL)

Hello dear forum, I noticed shortly after I bought my Xceed PHEV, that the rear right speaker is only half as loud as all other speakers. Because of the error he is now in the workshop. Kia Germany told the workshop what they have to test. They found nothing, quite the contrary, they still have an Xceed demonstration vehicle, also with standard sound system, and there it was exactly so. The rear right speaker is quieter than the left !? Lt. measurement, both loudspeakers should be defective, but it comes out music!! Something doesn’t fit.