Anyone know how I can use a mic from my PC so the person using the headset and hear through headphones?
This should work...
Right click the speaker in the sys tray an chose 'Playback devices'.
Highlight the HTC Vive speakers and click 'set default'.
Click the recording tab.
'Set default' on your PC's mic.
Click 'Properties'.
Click the 'Listen' tab.
Tick the 'Listen to this device'.
Adjust levels etc on the other tabs and click 'OK'.
That should work but be aware that the mic will always be open, so if you are using SteamVR to mirror the audio back out to speakers you may get feed back. But you can keep that 'Listen' tab open and just tick/untick the box and hit apply each time if you want to turn it on and off.