The master volume switch should still control
the master volume on the system, otherwise the switch should be removed if it's no longer going to be supported. See
this really old post describing the bug back when mIRC still lacked proper Vista support. I see this as a bug. /vol without -v should control mIRC's own volume, but with -v it should control the system's volume. Note that this *is* possible, and mIRC can do it.