No sound when playing videos

I can listen to music on my computer, but not when I watch videos on sites like Youtube.

I run Windows XP (32 bit) with Kaspersky internet security.

My browser of choice is Opera, but the problem seems the same with all browsers e.g. IE, Google Chrome and Firefox.

I have tried loads of things suggested on internet (including this site), but nothing works.  I have uninstalled and re installed Flash.  I have version 11.1.102.55

Can anybody please help?