The basic way to do this would be to route it through the pulseaudio server.
if you install pavucontrol (type sudo apt install pavucontrol in a terminal) then this will give you a control panel for volume control and redirecting audio - in theory at least.
You should then be able to send the output from CubicSDR to the analogue output, and use pavucontrol to set the FLDigi input to the monitor of the analogue output; I'm still working on that, though.
Another option might be by using pactl to install a loopback module - that's a bit more technical, though, and I'm still working on how to do it.
