MY INTERNET SOUND DOSEN'T WORK??????????

so i got on the compuuter the other giorno and went on Youtube and there was no sound then i went on downloads.nl and there was no sound but i went on itunes and windows media player and i could listen to Musica but i cant on the internet plzzz tell me wat to do to fix it!!! i am desperate!!!! and PLEASE dont tell me to check if all the sound bars r turned up cause i hav checked it a million times so...........PLZ HELP

thanx