I am using RCA panel mount connectors because thats what I have, you could make this all with 3.5mm stereo jacks, 1/4" phono jacks, or even a mix of different types! I want to give credit to Curt @ scribd for his inspirational writeup, I'm essentially adding photos, visual design and diagrams.
Currently I have my mac tower, xbox 360, and laptop connected to the same set of speakers with no problems. I wanted to make a small, non-powered box that looked good sitting on the desk.
Build Time: 1 hour, not including research + documentation
Total Cost: $15
$8 -Diecast Project box
$2.50 -Panel mount connectors, (six RCA female, one 3.5mm stereo female)
$2 -18 or 20 gauge wire, solid strand (this should get you 10-20 ft. I only used 8 inches total)
$2 -4.7k-ohm 1/2 watt Resistors
$.50 - Small Ruber Feet
Drill, 1/4" bit
Soldering iron, Solder
Electrical tape or Heat Shrink tubes
Knife or actual wire stripper
*optional* Dremel to thin out the wall of my project box, yours might be fine without
*optional* Alligator clamps to test connections before soldering everything together
Step 1: Detailed Part List Photos
The Diecast box actually worked out to save me about 20 minutes of soldering because it is conductive enough to ground all of these connections. Win.
Step 2: Drill + Screw + Tighten
You'll need 6 holes for the RCA connections and 1 for the stereo jack. Screw the connectors down tight!
Step 3: Circuit Diagram
Basically, all input lines have a wire and then a 4.7k resistor attached. Each side (all 3 Right channels, and all 3 Left channels) are then soldered together after the resistors, and another wire runs from that connection to the appropriate pole on the 3.5mm stereo jack.
Instead of soldering everything at once, I used alligator clips to check as I progressed...but essentially, once things are soldered together, you just need to tape or heatshrink the connections.
Then you are almost DONE!
Step 4: Finish!
Then you are DONE! Enjoy!