External Sound Cards

I recently purchased a refurbished computer that does not have a sound card. I do not want to add an internal card, because it will void my warranty, unless I pay someone to install the card, which I would rather avoid. I found an external card, which looks like a network adapter and plugs into a USB outlet. Given that I have almost no sound, is this a good option? I have seen them for as little as $4. Thanks in advance...