I've put Logitech surround sound speakers on my computer and now the sound doesn't work on the internet. Why?

Itunes and other music programs work but there is no sound on the internet. (Myspace, Youtube etc.) Please help!
I've put Logitech surround sound speakers on my computer and now the sound doesn't work on the internet. Why?
There shouldn't be any trouble with sounds from the internet, unless you have disabled the sound by mistake, or 'sounds' have been turned off in the internet options. Do give details of your version of web browser. The more details, the better.