Dell should have given you a CD that has all of those type of drivers on it for you to install at times like these... Otherwise, it would be part of your motherboard driver set.
Go into the properties for My Computer and then go into the device manager. Under sound devices it should give you your sound card thingy...
*EDIT* Sorry, that's under the hardware tab when you go into the properties. And it's listed as Sound, Video and Game Controllers.
You can download it, you can't use it on a Mac though.
If your sound driver isn't installed dxdiag wont have a sound tab. There's 3 entries in the Dell support site linked. If you don't know which one it is, the service tag link (given after the page with all possible Dimension 5150 downloads) will taylor the downloads to what you have. If you don't know your service tag, you can find it in the BIOS, Dells have a section for it off the main menu.