I have a DVD movie to watch on my laptop, but for some reason there's no sound.
Can anyone tell me what could cause this, and how I can amend it?

It depends on which 'Media Player' you are using. It is p[robably that you are missing a codec for sound. It is probably the AC3 filter that is missing.

The best way to sort this is to use software like Power DVD, this installs all the nessesary Codecs and stuff for playback.

You can download a trial version here.
http://www.gocyberlink.com/english/products/product_main.jsp?ProdId=28

Its the program I use. (got a free copy with my DVD Writer.)

ASll this advice assumes that you get sound in other applications like games and stuff, if not is the drivers for your sound card you need to update.. ;)

Have you tried the DVD in a different player or tried a different DVD in your laptop?
Speakers could be disabled/broken - have you tried headphones?
Could be a problem with your soundcard
Could just be the settings on your laptop - check that sound is enabled

The latest Windows Media Player 9 (and all updates) will play it. As will Cyberlink Powerdvd 5. I had V4 and had no sound - like Azrael said it's a missing audio decoder.