To monitor from up to three sources simultaneously (instead of the default single source), first enter setup mode. To enter setup mode press the mono and alt spkr buttons simultaneously. Both switches will flash alternately indicating that the D-Box is waiting to set the input selection mode and the analog monitor gain. Then press the sum switch. It will light up. Next, exit setup mode by pressing one of the flashing switches and like magic, monitor a combination of inputs mixed together. When in setup mode, pressing the lit sum switch extinguishes the light and returns the D-Box back to normal monitoring mode. (i.e. one input at a time.) So for example, monitor from the sum switch, analog switch and either the daw or cd switch. (The digital switches will not combine).