Callum's suggestion may work it's certainly worth thinking about. You do need something to amplify the output from your laptop and you do have a few options. Let's have a look at them!
Option 1 would be to use software to increase the gain/output (we'd say "amplify"!) of the signal. There are many software packages out there that can do this. Very few are any good. You are more likely to end up with a distorted sound and putting a distorted signal in to your mixer can cause further problems. However, a lot of them are free and you may find one that just about does the job. I'd start by looking here:
http://download.cnet.com/1770-20_4-0...type=downloads
There are lots of paid-for audio enhancers that may include volume increase options. Google is your friend here!
Option 2 would be Callum's suggestion of the headphone amplifier. As with the software, you'd have to be very careful of not amplifying the signal too much and distorting it as, again, a distorted signal in to your mixer could cause problems such as excessive clipping. Have a look at this page as a starter for ten! :
https://www.google.co.uk/#q=headphon...ifier&tbm=shop
Option 3, and this would be my choice, would be to invest in a new, and more powerful, soundcard. Search Google for soundcards, read reviews and see if any suit your needs. I use a Creative SoundBlaster USB soundcard and it is a massive improvement over on-board sound. I've had it for years and it's still going strong.
Oh, and good luck in your quest!!