What's the best way to mute summed stereo channels?
You will want a cap in series with R3, and usually muting does better switching to ground as a shunt rather then in series as you have it shown, or do both which will help with the attenuation given analogue switches rather poor on state resistance. 

There is much discussion on click free mute circuits in Selfs "Small signal audio design" which I highly recommend to anyone designing a mixer, it is basically a masterclass on analogue mixer design, well worth the price.