Some sound but not all :S

So basically I have some sound on my computer but not all, when I'm on certain sites like FB my games don't have sound and when I'm playing the Sims theres no sound there either but I can still listen to music and watch videos, and my MSN and Skype sound still works as well, and in the volume tab in the bottom right hand corner I click on "mixer" and all the volumes are turned up but still no sound on my games which tends to be really annoying. Any help will be appreciated