Line 1 Out Second Slice Muting?

I use Line 1 Out for the digital modes. The WSJT-X software that I use for JT9 and JT65 includes a split option that allows both modes to be worked with the same GUI. Split mode with the Flex-6700 requires that a second slice be launched. In this case, I assume that the received audio from both slices is present in the Line 1 Out signal. Since Slice B is only used for transmit with WSJT-X, I want to mute any audio from that slice. Some of the documentation (on this Community site) makes reference to a "Mixer" which I assume would allow me to control the audio output from any slice. I also assume that the slice mute available in the slice control panel only mutes audio to the headphones and/or speaker. My question is how does one access this mixer in 0.13.10? If it is not accessible now, when is its implementation planned?