There are many different models of Acer Aspire notebooks, you are not being specific enough with your information.
Off the top of my head, you might have your volume muted in Windows. If you have something plugged into your headphone jack (or stuck in it), this would also cause the built in speakers of your laptop to not work. Finally, installing the wrong sound drivers or incorrectly installing them could also cause your sound not to work.
If none of these things get sound working for you, I suggest contacting Acer via their toll free number they included with your laptop when you purchased it for assistance.
