I also don't know the cause but for solution is to set this to environment variable before running video call application
For convenient, you can rename the original application bin file to other name and making below script naming as bin file.
For example, I did rename my /usr/bin/google-chrome to /usr/bin/google-chrome.real and create a script at /usr/bin/google-chrome with below content
Now Gmail video call on my Google Chrome should work. (Skype also can do the same way)
Tested with Ubuntu 10.04 on 32-bits machine.
Credited to some forums in internet. Sorry for forgotten.