Really a note to future me. I'm using an SDRPlay with the excellent SDRuno software on Windows. (Why isn't there comparable software available on Linux?). The trick to getting the audio out of one piece of software and in to another is to use a Virtual Audio Cable (VAC). I paid money for one some years back but it's not as good as VB-Audio Virtual Cable from the excellent folks at VB-Audio.
The deal is that you get one virtual audio device for free and can pay money for additional devices.
Here's how the output is set in the program that produces the audio we want to decode:
Here's how the audio input is set in WSJT-X:
Note that although I've set an output it's not used with a receive-only SDR.
And here's the full configuration with WSJT-X decoding WSPR on 20m. (Click any images here to enlarge).
Third party Windows time synchronization software no longer needed.
I don't know when this changed but it used to be that to decode WSPR on any version of Windows you needed to install third party software to sync the clock well enough. This appears to no longer be the case at least for me running Windows 10 Pro, Version 1709.