- apt install pavucontrol
- start your applications (gqrx/fldigi/direwolf/whatever)
- execute pavucontrol
- In the "Playback" tab, make sure your "output" app is showing up and bouncing the sound meter
- In the "Recording" tab, make sure your "receiving" app is showing up. Click the sound card button and select the "monitor" entry
- Should start working immediately.
For FLDIGI to show up it needs to start recording from the sound card, I had to go to Configure > Sound Card > Audio > Devices > select "PortAudio" > leave capture and playback at "default"