Simulation of circuit for 5.1 summing box, with two channels active. The green line a probe on the level meter. It tracks the loudest channel at any given time. I didn't originally plan on having a level meter, but I decided that the user should at least be able to check if any of the channels are in danger of clipping, since this thing is USB-powered and doesn't have a lot of headroom. Do consumer sound cards generally stay below 1.41Vp-p on their line outputs at max volume?