... that's the headset speaker, not the microphone. Microphones are audio *input* devices. OK, now that we're clear on that...
Please remember to search before posting. Also remember to post in the correct section. This is a question; it belongs in Q&A. If you were offering an app or library or hack of some kind, you would post it here.
To attempt to answer the question... looks like you either need to use
if you're working in managed code, or use GetDefaultAudioRenderId
and specify the value Communications from the AudioDeviceRole enum. Note that doing this may require that your app declare the ID_CAP_VOIP capability.