You can find your motherboard from the very first screen of your PC. When computer boot, at the first screen, press PAUSE key. At the top it shows the Processor, RAM etc. of your Computer. At the bottom, it shows a date and a long no. in 3-4 parts. The first part of this no. is the model no. of your motherboard. Next parts are the manufacturers information. Your can get a list of these codes and details from the internet.

Or what i just do when im too lazy, just open your pc case and look on the side, borders of your motherboard.
You should see the brand and type number there.

once u've gotten the brand/type u can go to the official site of
your motherboard brand and there u will be able to download the appropriate drivers,

IMPORTANT:

this is why u need your type number too. If you download drivers with a different type
number you'll still have some problems.

To make this a little more understandable ill give an example:

My motherboard is from gigabyte, so i go to the official site of
gigabyte. then id have to go to motherboard product type.
and select the type of my gigabyte motherboard.

Then there's a list of the latest drivers.
Note: make sure uve gotten the correct driver for your operating system
( for example if u have vista x64 dont download the vista x86 )

For gigabyte motherboards the audio driver is called Realtek.
when uve found the correct drivers just download and install them.

Added (2008-12-29, 5:18 Pm)
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If you have a seperate sound card, just do the same and
download the latest driver of it.