Q: connecting IPhone 7 to Kurzweil piano keyboard

Hi, I am connecting my IPhone 7 to piano keyboard through USB midi cable. Once it is done sound from iPhone apps goes through piano which I do not want. How can I change that back, so app sound goes through phone.

Once phone is connected to piano I cannot change volume on iPhone, side buttons and toggle in settings is not active. Also when you swipe up and then try to select source for audio there are only other devices to be able to select not iPhone itself.

pleas advice