How to control the USB Audio device with IOS app

I have a USB Audio Soundcard, it work on ipad and iphone well. Audio can be play through the soundcard. Now, i want to send some controlling command to the soundcard. I think it will be done with the USB HID interface. But I do not know how to do it. Can anybody give me some guide ?

My App is designed for IOS, not Mac OS. The app can send volume and mute command to the soundcard, which are the Usb-Audio-class request.

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Have you solved your problem? I also need to connect the USB audio device through IOS app, but I don't know what to do. Can you help me?