So just after a software update and reboot, I found that the Applets panel (Raven, right?) changed. Now, it doesn't list all of my sound devices - it lists my USB Blue Yeti microphone, but not the Line Out device (my speaker output port). I can still select the device from the Sounds window. Attached is a picture showing the issue.
On the left is the Sound system preferences dialog - I have selection of the four available sound devices there. On the right is the panel, which only shows two of them (and incorrectly shows the Output as set to the Microphones headphone out - it's set to Line Out, as you can see on the left).
Freshly updated Solus. Thanks.