I'd also be curious what app/program you're using to watch the DVD, while it's been awhile I've had some players go louder than others, or VLC allows you to go 200% and may provide some amplification.
Is it just DVD's or is it all sound(hard to give the best suggestion without clear definition of issue). I've seen some laptops that simply come with super cheap speakers so if normal noise or music doesn't get louder either you may consider getting some USB speakers/headset.
I agree with checking/re-installing recent sound drivers as a start though. Also if you have Windows 7 the sound controls are a little different than they were in XP/Vista so you'll want to make sure all the options are maxed.