When switching between Ableton sets, or other laptop dead air times, I want to be able to bring up ambient noise to fill the gap. I have pre-recorded sounds on a SanDisk mp3 player, which I want to blend with the output from a Mac laptop using a foot pedal. Usually I combine the two signals through a mixer, which works well and provides more flexibility, but to travel light I want to trim this down to a very small passive mixer. In short, I want two 3.5mm stereo audio sources as inputs going to one 3.5mm stereo out for the amp. I'd also like another 3.5mm stereo out for recording, which needs to be attenuated to mic level.
My first pass at solving this was to build a passive summing cable, then a stereo Y plug with one branch going to a pad cable for recording. That worked, but it was a bit clunky, and was plagued by occasional cutouts and other issues related to treating the computer output as TRS when it is actually TRRS. I've given up doing it as a cable, but it does basically work, so if you want to try it that way you can check out that writeup.
Here's the wiring diagram for the mixer. The summing portion is on the left, and the Y-plug/PAD part is on the right.