All kinds of speakers can suffer from a low-frequency buzz, commonly referred to as a "hum." There can be many causes for speakers humming, and some can be easier to fix than others.
A hum coming out of computer speakers is usually very low frequency, around 60 to 120Hz. Some people may not even be able to hear it, and it isn't always noticeable. Most often it can only be heard when no other sound, or a very quiet sound, is being broadcast by the speakers.
Sometimes humming from speakers is just caused by a loose or partially plugged wire. Check your connections and make sure everything is plugged in securely.
Ground Loop Hum
Ground loops are common causes for speaker hum. A ground loop is interference caused by other electronics sharing the same electronic current as your speakers. You can diagnose a ground loop by unplugging other electronic devices around the computer. If the hum vanishes, the problem is most likely a ground loop. These can be fixed by buying ground insulators and interrupters, which are available at most electronics stores.
Computer speakers are usually not shielded to be protected from radio frequency interference. Radio stations, cell phones and computer monitors can all emit radio frequencies that create speaker hum. Moving the speakers and wire cables can sometimes eliminate or minimise this problem. More often than not you'll need to get new speakers with better shielding against the interference.
Speakers, like everything else, degrade over time. Speaker hum may sometimes be caused by deformed speaker coils, a blown speaker, or any other deformity or damage of the speaker. Speakers usually cannot be repaired. In these cases, the easiest option is to replace the speakers.