The volume control buttons on Vaio notebooks control the System Volume slider by default. Programs can install themselves and hijack the controls to change other volumes. For instance, if it's a Media Center PC, then the buttons control MC's volume (but only when MC is running), if you have Winamp, the buttons can be used to control only Winamp's volume, and not the computer's.
Perhaps the webcam's drivers you installed are hijacking the hotkeys?