Connecting cell phone and computer audio through TRS and TRRS 3.5 mm audio jacks

Question

I am trying to connect the computer audio output to the cell phone audio input and vice versa. There are two connection designs because I need to be able to connect both to a desktop and a laptop which feature different audio interfaces.

Regarding the connection I have several questions:

1. Are the pins correctly connected?
2. Will the signal levels work as is or should I amplify/attenuate them?
3. Can I only connect one channel and make away with the mixer? Obviously whatever comes out the other channel will be lost, but I am interested in knowing if it is safe to directly connect the audio output of one channel to mic/line in.
4. Can you think of a way of mixing the audio channels with only passive components or removing the mixer in any other way?
5. The desktop has both mic in (pink color) and line in (light blue color). Which of these inputs is better suited for my application? What do I need to do differently if I connect to one or the other?

Many thanks in advance.

Answer 1

1. They seem to be correctly connected
2. Mic levels are much lower than line levels. Some audio interfaces have selectable gain control to switch between line level input and mic level input, some don't. Output volume can be decreased to match mic input. It may be a good idea to attenuate the signal for the mic input.
3. Yes, it is safe to use only one channel and connect directly, except for the attenuation for mic input.
4. Simply use two resistors to mix audio channels together. Add third resistor for attenuating the output.
5. You can simply plug a direct cable from line out to line in, that's what it is for. The voltage levels match and you get a good quality two channel stereo link, as mic inputs tend to be single channel mono and more noisy due to lower signal level and higher amplification gain inside the PC.