Audio question

I posted this to the iPad user group and didn't get a response, hopefully someone here can help.

I have a USB microphone which I use with my iPad Garage Band and works very well.

I want to use it to get better audio on videos I need to make but when the microphone is plugged into the iPad I get no sound recorded at all while shooting video.

I've looked for something to toggle or switch or click on but find nothing. Maybe it's impossible?

I'd very much appreciate any suggestions.

Thanks

Mark