uC Latching Relay True Bypass - nearly!

[Transmogrifox]

I suggest a tightly located 0.1uF cap for decoupling high frequency noise on the uC itself, then a good several 220uF bulk caps to supply relay switching current.

How are you connecting the 220uF caps?  Is it one on each of the two relay connections?

The big caps go across the power supply to which you connect the relays.  In the context of the original post, if you are driving the relays with transistors, then you should have the power supply decoupled right at the places where the transistors connect to ground.  The idea is to reduce sudden switching currents from making waves further up the power supply system.  You supply charge locally to make most of the transient switching current come out of the caps and not drive current through your whole grounding system and/or bounce your 9V rail that supplies the audio circuits.  You may even want to put a resistor in series with your relay switching circuit that is as big as you can make it and still power the LED and uC.  Then a sudden change on the bulk capacitor discharging into the relay won't result in a very high magnitude surge current up the power supply.  It will only dip locally then trickle-charge for the next switching cycle.

This is only one of the ways a pop can get into the audio path.  Other things as mentioned by RG is transfer of charge from one part of the circuit to the other when switched in.  There as mentioned before you have to be careful all contact points are discharged to 0V so nothing jumps when you switch over.

Also the mute circuit adds another piece of complexity since the logic switching the muting element can couple through to the audio path and defeat the purpose of muting.

I hope that makes sense without a schematic.

[MetalGuy]

Just to let you know that I tried the VO1400 (from Vishay, Ron 2.5Ohm) which is similar to TLP222 (but SMD) in another setup and it works fine. I tested it with 33V RMS sine wave and it showed no signs of clipping. My DMM couldn't measure the capacitance which means it's lower than 10pf.