Computer Speakers Not Producing Any Sound

Recently, I bought Altec Lansing IML247 Orbit USB speakers for my computer. After 3 weeks, I’m starting to have issues on using it. The problem is I can’t hear anymore sound from the speakers. Despite that, I can still listen to music by substituting my speakers with headphones. I wonder what is wrong. My computer is up to date but I still can’t find the reason why the sound is always muted when I’m using those speakers. I am running a Windows Vista computer. To those who can help, a thousand thanks to you.
