I have noticed if I leave the system unattended for a period of time and the screen saver is initiated, the announcement audio will switch to USB sound card in my IC-7300 and transmit the announcements. I have noticed announcements on other stations transmissions, I wonder if this is from the same problem
Probably your main sound device goes into sleep mode. Change the Windows power options or use a utility like https://www.softwareok.com/?seite=Microsoft/DontSleep while VarAC is running.