My system is a MBPro, running Snow Leopard 10.6.4 and Microsoft Messenger 7.0.2. I think this is just a bug in Microsoft Messenger for Mac. The scenario:
I have the Griffin iMic USB sound interface, and I've been hoping to use it in Messenger as a headset for audio meetings (which my company likes to do on Windows). The headset I have plugged into it works just fine with other applications (e.g. Audacity can pick up the audio). When I attempt to use it with Messenger, it only picks up audio via the built-in mic.
After some experimentation, I've discovered that Messenger will use the USB audio input device ONLY IF it is plugged in and chosen in sound preferences BEFORE starting Messenger. So I believe Messenger is determining the audio devices to use at startup and isn't checking with current system preferences later.
I only use the headset for meeting purposes, so it isn't usually chosen as the current audio device. So the typical use case is to have someone contact me via text on Messenger, then suggest an audio call. At that time I connect the iMic, and change the audio preferences so that it is active. Then I have to restart Messenger to get it to pick up the change.
So I'm wondering if there is another way to get Messenger to respect the System Preferences. Restarting Messenger isn't terrible but it is a bit jarring to the other party that I'm trying to conference with.