Taken from the tv license web site
http://www.tvlicensing.co.uk/information/index .jsp
Do I need a TV Licence?
You need a TV Licence to use any television receiving equipment such as a TV set, digital box, DVD or video recorder, PC, laptop or mobile phone to watch or record television programmes as they're being shown on TV.
If you use a digital box with a hi-fi system or another device that can only be used to produce sounds and can't display TV programmes, and you don't install or use any other TV receiving equipment, you don't need a TV Licence.