Mixing analog audio signals with resistors
I think you'll need more than just resistors to do what you are describing properly. You should probably go active. You could try a simple summing amplifier like this:



This will invert your signal but you can always add another simple inverting amplifier stage at its output (basically the same thing in this schematic except only using 1 resistor at its input).

You will have less loss this way and you can even apply gain if you want to (determined by Rf/Rin). If you add a potentiometer for each input, you'd have individual volume control for each one. You can add more inputs if you like as well.

Hope this helps!