A lot has been tried so there is not much left to try that I know of. I'll add what I can.
1. Go to this FLASH AUDIO TEST PAGE to see if Flash sound works and that you have the latest version.
2. Check the Youtube and itv player to make sure the sound volume is not turned down or muted on the players themselves.
3. Go to the Yahoo Messenger Options and check your sound settings there. If you can't hear audio in Messenger chat make sure that sound level is not turned down or muted.
4. To determine whether or not this is an IE problem or setting download and install Firefox (en-US link) and try Youtube again. (Note you may be asked to install Flash for Firefox)
5. When you uninstalled IE8 did web sounds work in IE7?
6. From the Control Panel Add/Remove uninstall all Java and Java updtates. Reinstall Java.
7. Uninstall Adobe Flashplayer and plugins for it. (Note you may have to reinstall Flashplayer for Firefox). Also in Explorer navigate to Downloaded Program Files and by right clicking the properties of the Active X files look for the Flash related components and remove those too. Reinstall Flashplayer AFTER a reboot.
Or better yet run the Adobe Flash Player uninstaller before reinstalling. Reboot first.
8. Uninstall the itv player in case that is part of the problem.
To me it seems to be a Flash and or Java issue but possibly a third party program is the cause. Whether it's an IE add-on or something else is hard to say without knowing what is on your computer.
Anyway that's all I have for now.