100£ is low. Instead of getting a brand new laptop, you should get a refurbished or used one. There are plenty of them on ebay.
Here are a few exemples :
http://www.ebay.fr/itm/HP-Probook-6530b-/322379535827?hash=item4b0f5155d3:g:aiMAAOSw44BYbL1~...
That "green plug" is called a jack.
Are you sure you are plugging it on the "input" port ? Are you sure you have connected your speakers and turned it on ?