Source selection returned the first monitor PulseAudio enumerated and used the default sink name only to report whether that guess was right. A USB microphone that also exposes an output enumerates ahead of the real speakers, so the chosen monitor was one nothing ever plays to and the system channel recorded digital silence while warning about it every few seconds. Prefer the monitor of the default sink, falling back to the first monitor only when there is no default sink or no monitor matches it. Announce a selection only when it changes, so a steady desktop stops rewriting the same two log lines every tick.