MOSFET based audio switch generates thumping sound
10nF capacitor C8 charges and discharges substantially from the audio when the mosfets are ON due the low time constant. The voltage in the capacitor is unpredictable. When you turn the mosfets OFF C9 is charged from C8. When you turn again mosfets ON C9 discharges and creates the thump. 

Try to use mosfets so that turning them ON or OFF doesn't change capacitor charges. For ex. remove the capacitors from the signal path totally or at least remove C9 and have much bigger C8.

Another approach is to have a fet in series with the signal, not shorting it to the ground. Driving the fet becomes more complex but it's possible.  I have built a multichannel noise gate with piezo triggers for better separation in drum recordings with that principle. In that environment the extra sounds were treated differently (=nice new click in the bass drum! make it louder!)

A light dependent resistor can be a better and more easily manageable choice if you try to increase the series resistance instead of shorting the signal. You can assemble a led and LDR in a common tube or purchase LDR output optocouplers.