hmkay, nothing suspicious, and if you say you didn’t change anything in between “last week” and now (actually you didn’t answer that question)… I guess it’s some control under the hood which is messed up.
When the Talk button is not pressed, the only way to hear Mic1 in the main mix is --IIRC-- if
[Microphone],master is set to 1. Via GUI this is not possible with the officia skins, also it’s (should) be reset to 0 every time Mixxx starts. And it can only be toggled by a controller mapping or some custom skin (or some custom keyboard mapping).
If you like to find out if that’s he case you need to
start Mixxx in developer mode
16. Appendix — Mixxx User Manual
open Developer menu, Developer Tools action, Controls tab.
[Microphone],master into the search field at the top (this is for Mic1, for other mics use [Microphone2] etc.).
check the value in the the third column