After my latest re-installing of Windows XP, and after installing the latest Nvidia drivers for my nForce2 chipset (on a Shuttle SN45G mobo), it seemed that the microphone had died. I then managed to hear it played straight through to the speakers, but the signal was never registered by the nvmixer app and stuff like MSN Messengers' audio/video configuration wizard.

I finally got fed up with this situation this morning since I do have real use for the mic (a sort of long-distance baby-sitting), so this issue's not just a beauty flaw. The Shuttle FAQ for the SN45G does acknowledge the problem, but the solution isn't correct in my case; I already knew that the selected microphone has to be the Microphone2 option of nvmixer. After having tried my usual sources, the nForcersHQ's SFF and nForce audio forums and failed, I meticulously went through each setting in nvmixer, and thereafter the annoyingly duplicative Windows Control Panel Audio settings.

Once again, the flaw was at the Redmond end. After having activated the advaced settings (lisäsäätimet and lisäasetukset in the Finnish version of XP), the detailed settings window for the main volume revealed the problem: the Center speaker and subwoofer connected to Microphone -option (in plain English right at the bottom of my Finnish settings window) was ticked, and immediately after having unchecked it, my microphone began working.

Case closed.