See the start of the Hardware volume thread where @occip did this with input from @Ben007. It covers the connections to the XTAG debugger which you can get fairly cheaply from suppliers like digikey.com. You’ll need xTIMEcomposer and the USB Audio 2.0 reference implementation as a starting point, and the Tone Board schematic to see which pins are connected to what. From there it depends on your 8 channel source. That may require i2c or spi to configure it as well as the i2s for the audio data. I’m saying this from a theoretical point of view - I’ve not tried it myself.