This could be caused by many things including that the microphone itself has either stopped working or if you just purchased the unit it may be defective. but it can also be caused by a improper installation in windows itself.
to reinstall the camera please follow the steps outlined below.
First step i would unplug the camera and uninstall the current drivers.
2nd i would follow this link to Hp's website
where you can download the latest HP pro webcam driver/software.
3rd after downloading the software do not plug the camera in and restart your computer.
4th. After your computer has restarted and all the normal startup processes are running. Open the newly downloaded drivers for the HP pro camera. install the camera. After the installation is done then you can proceed to plugin the camera. Unless you were prompted to at a specific time during the installation.
After that you should be done and it should be installed and working.
If the microphone on the webcam is still not working it is most likely that the microphone itself has stopped working. If you just bought the unit you should return it for an exchange. If the product is still under warranty you should contact Hp and get it fixed under warranty.
If you need further assistance please feel free to follow up with a comment and i will be here to assist you. If this assisted you please feel free to rate the solution. thanks and have a great day.