My web cam is A4 Tech Pk-635M ViewCam Pro.How

Not sure what messenger you are using, so will give you two fixes and if you need the other let me know.
Normal 0 See section B. Yahoo Messenger in the info below to setup the Audio & Video.
Also send this to your family and friends so they have their computer setup properly.
Web Camera’s have to be setup on the program they are to be used with. Just installing the camera on your computer does not set it up to broadcast on messengers or Skype.
Each program has Options that have to be setup in order for the Audio and Video to function properly.
A. Skype Setup:
After downloading and installing Skype (http://www.skype.com/intl/en-gb/download/skype/windows/beta/downloading/) set it up as follows:

1. Go to Tools, then Options, and click on Audio Settings and set the Microphone and Speakers to the computer Audio Card.
2. Then click on the Video Settings.
3. The camera if Installed Properly will come up and show a video picture.
4. Then, on the bottom of the page click on Save.

The camera and audio should function properly. Your friends and Family will now be able to see and her to you.
B. Yahoo Setup:
After downloading and installing Yahoo Messenger (http://www.soft32.com/download_30.html ) set it up as follows:

1. Go to Messenger at the top left and click on it.
2. Next go down to Preferences and click on it.
3. Next Calling and Devices on the right and set the Microphone and Speakers to the computer Audio Card.
4. Next go to the Test Audio section and (in blue writing) click on “Open the call setup assistant” and follow the instructions on the left window.

Hope this helps. Bud