Depending on what you plan to do with audio, the on-board AC97 based sound might be all you need.
Assuming the hardware is fine, you need to install the audio driver from Asus or VIA, and possibly enable the on-board sound in BIOS setup.
If you are looking for something with more features than the on-board sound, or plan to do recording work, you may want / need a better sound card.
For serious recording work, you need a true audio A/D converter card and not a standard sound card.
For just better performance, and more features, most of the $10 cards you can find on the web work just fine.
For a general purpose sound card capable of capturing LPs and cassettes to transfer from analog to digital, I like the cards offered by Turtle Beach.
Chas