The only actual audio in, is the (old school) mono mobile phone takeover function. You can send mono audio to the back panel of the head and pull a line to ground - the unit will mute CD/Radio audio and put your mono audio in the left side speakers only.

I use this function at the Dragon to cause my FRS radio to mute the car's system so I can hear the FRS radio clearly and loudly.

But unless you buy Rudy's product, you better be good enough to work up your own interface like he did! 'Cause there IS no other "Aux Input".