MUT function can be used for main or VAC sound card independently. Check MUT and right click with mouse over MUT button. From context menu choose which channel should be muted (initially Both).
VAC audio output can be muted from GSDR main screen:
In Setup->Audio->VAC select which VAC hannels should be muted:
With little effort received Left audio channel can be sent to external program for Digital modes (like HRD or WinPSK) via VAC interface with right channel muted. During that time on main sound card muted is left (so-called digital signal) and received right ch. audio (CW):
Goran Radivojevic YT7PWR