I2S audio output on BBB

Hi, I have a beaglebone black working fine with an external USB audo adapter. I heat that I can use integrated audio on the beaglebone black to drive pins in I2S, I see the mcasp devices listed using aplay -L. Can someone give me a hint as to how to connect these to pins for I2S output? The next question can I send audio through a virtual mixer to the USB audio device and the I2S pins?

Thanks,

Jerry