Hi,
Did it happen all the time or sometime?
Please check online browser status:
Click Volume icon in the taskbar, click Mixer link button as below:

If it's fine, follow this guide to run troubleshooter to detect and fix the issue:
Tips for fixing common sound problems
http://windows.microsoft.com/en-in/windows/tips-fixing-common-sound-problems#tips-fixing-common-sound-problems=windows-7
Meanwhile, this similar thread also could be referred:
https://social.technet.microsoft.com/forums/ie/en-US/a4a1cfe5-93a5-4c0b-9bf6-f7db0304f2ba/no-sound-on-youtube-or-any-other-webpage
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