First be sure that your volume button at the bottom of your YouTube video is on high.
YouTube is based on Flash. Are you talking about online games that are based on Flash? If you Ctl-click the image on you game and you see a Flash menu, then it is Macromedia/Adobe Flash. if all of the problems are related to Flash, you might need to install a Flash upgrade. If the games are not Flash, then I don't know what would cause the problem.