Using Mac as external audio amplifier, while routing its output to USB?

I've got a Yamaha mixing panel which accepts my Mac's audio output via USB. Great. Now I would like to pipe the output of the mix back to the Mac's input, and have its speakers play the sound.

Is this possible?

At the moment, as soon as I select the Yamaha mixing panel as sound Output, the built-in speakers go grey, suggesting that they are not used.. but the setup I'm after needs them to be used, but in a very non-standard manner.

If it's not possible, then I "just" need to hook the mixing panel to an amp and speakers, and I'm trying to avoid that.