In most cases (all that I've encountered with Dell's) you need Dell specific sound drivers for everything to work properly. And, XP drivers are not compatible, you need Vista/Windows 7 drivers.
There is a special Alienware Owners "club" support section on the Dell forum. I would post there and see if those familiar with Alienware systems can help. Alienware Club - Dell Community
Also as this is an older Alienware system, it may not even have Win 7 compatible drivers and what you have is what you are stuck with. The Alienware site, for the older systems, such as yours, does not list anything either.