Windows 10, laptop, external audio interface, no playback - solved
I'm posting this so that if someone else is in the same situation I was in searches they can find this solution. I am on Windows 10, neither Musescore 2 or 3 were offering playback, no synth window and the portaudio options didn't yield any values. I am using an iConnectivity Audio 4 plus external audio interface. This is setup as my default playback device in Windows.
I had to make my laptop's on-board sound the default playback device (with the exclusive use box unchecked; my external interface does not even have the tab for this setting) to get Musescore to recognize any audio at all. I could then change the Musescore device to my interface.
I will attempt later to determine if subsequently resetting my interface as the default Windows device and restarting Musescore works smoothly and will report on the results. I have some work I want to get done first though, after this long, frustrating day of troubleshooting.