no sound in online streaming videos

hi,

suddenly my comp has stopped playing sound for the online streaming videos. the video and everything else is fine just the sound is not coming. i have checked all the mute buttons and other normal settings, all are fine.Also, the notification sound in gtalk is also not working. no sound is coming on alerts.

the whole point is no sound for all internet related applications.

i dont know what to do other than formatting my system.

do the internet browsers have a different setting for volume.

kindly tell me a solution.