Passive Signal Summing/Mixing, Using Only Resistors (ECE Design Fundamentals, Georgia Tech course) https://www.youtube.com/watch?v=vbr4XbAhlnQ

• explains a bunch of reasons why generic resistors are super dog shit for designing a mixer, mostly because each resistor on each signal will influence the gain of every other signal, no independent control

Active Signal Summing/Mixing, Using Op Amps in Inverting Configurations (ECE Design Fundamentals) https://www.youtube.com/watch?v=k4B43nhb91I

• use OpAmps to mix analog signals together, describes the inverting configuration in this case

• gives you independent control of the gain of each signal through its own resistor (can adjust the gain of signal V1 by changing only resistor R1, and can change the gain of the whole opamp by changing Rf)

• dont forget your opamps should have super high gain (ideally infinite) so that the feedback works properly, the actual “gain” you get is by feedback not by inherent opamp characteristics

• use a differential op amp to convert differential signals into single ended signals ?

Convert Single Ended signal into Differential Signal https://www.diyaudio.com/community/threads/how-to-convert-single-ended-audio-signal-into-differential-signal-single-power-supply.385108/#post-6995371

Muting a Circuit (more complicated than you think)

https://sound-au.com/articles/muting.html

• remember that when using something like a transistor that the transistor is effectively a diode, and audio signals fluctuate between positive and negative, so you need dual MOSFETS
• Optocoupler Transistor

Gain levels (dB, dBu, dBV)