My volume want work now for some reason???

Hello,
i need so help with my volume i have checked everything, it works with my limewire i can hear my muisc but when i get on youtube or any video network i can't hear nothing its says my volume is up which it is but when i try to play videos {example: youtube} i can never hear nothing, i have went into the control panel an all of that but still nothing SO PLEASE HELP......