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.
I found a solution: there's a free little OS X utility called LineIn (written by Rogue Amoeba Software). It just lets you route any audio coming in on the line in connector to any destination, including the built-in speakers.

So now I've got Garagaband routed to USB Audio out.. to the mixing panel, and then its output is piped back into the Mac via the Line input, the LineIn program, and finally to the built-in speakers.