No sound from iPad Pro to UR44c

I have just upgraded from a UR22C to a UR44C (because the UR22 didn’t send midi clock). I have the UR44C set up but there is no sound coming into it from my iPad (have tried from cubasis and also from other sources such as SoundCloud). The UR44C is recognised by the iPad, midi works fine, and the sound from external synths comes through fine). How can I solve this?