Problems with earbuds and the ipad pro (12.9)

I have a new ipad pro (12.9 in) and cannot get any earbuds to work. I have plugged in earbuds with a usb-c connector—nothing (although the volume controls work). I have used an apple dongle 3.5 mm to usb-c adaptor with apple earbuds—nothing. What´s wrong, please? Help!