Are sound cards only supposed to be used on desktop PCs, or can you
use a sound card on a laptop, more specifically and alienware 51-m
laptop.

No, usually sound cards are intended for desktop PCs; laptops
generally have a built-in sound card which is soldered to the
motherboard.  As far as I know there is no way to run wires from a
laptop to the pins of a sound card and have it function.

Laptops in general tend to be more "proprietary", meaning that
although they work similarly, the manufacturers each might manufacture
them very differently from each other.  As a result their components
tend to not be interchangeable, and the laptops themselves tend to not
be easy to do modifications with like you can more easily do with
desktop PCs since they use more generic parts (e.g. separate sound
cards, video cards, other stuff).

While you can't add a 'regular' sound card to a laptop, you can get
higher quality, multi-channel sound out of your laptop by adding a
external USB sound card.  One example is the Soundblaster extigy:
http://www.soundblaster.com/products/extigy/
Just do a search on 'external sound cards' and pick the one that best
suits your needs and price range.

- Davis

thanks guys, the soundblaster looks good- but is it too bulky?

Try the following review of 5 external sound cards:
http://www6.tomshardware.com/consumer/20040223/index.html
If you want small, the Hercules Muse Pocket should fit your needs - it
is barely bigger than the volume control knob.

- Davis