I am trying to mix a mono microphone into both channels of stereo audio. The output of the microphone amp is yellow. The stereo in is red. I am using 2 schottky diodes in order to mix. Is this an imporpor way to mix audio/is there a better way? Also, this device is low power, but I am using a lm386 to amplify the electret microphone. Are there better alternatives or am I doing it right?
No, using diodes does not accomplish what you want. That will partically rectify the additional signal, making it sound like a mess.
You need to actually mix the new signal onto each of the two stereo signals separately. That might be as simple as a few resistors if the new signal is well buffered so that its impedance is low, and whatever is receiving the mixed signals can handle high input impedance, like a few 100 Ohms.
Feed the mono output from the amplified microphone signal to left and right channels with a resistor such as 1 kohm. It can also be as simple as shorting both left and right inputs electrically to the common mono signal but using a resistor is safer.
The lowest power solution would be to not use an amplifier - you may indeed get away with connecting the microphone output to left and right input channels. From your "in" node - connect it to left and right.