Yes you can add speakers to your PC. There are self powered speakers that can be connected directly to your sound card with analog or digital cables. You can also feed these same outputs to a stereo or home theater system. If you want audiophile quality you would connect to something like a Peachtree Audio to your PC and add any quality speakers that you like.
A good set of active ( powered ) speakers will not only boost your sound coming from your computer via your sound card but will also improve the sound quality you hear --especially when listening to music