There may certainly be an issue with the built in microphone on your computer, but the problem may be a simple driver issue.
The following steps should reveal whether it's a driver issue or not
1.-Click Start>Right Click "My Computer">select Manage, a new window should open
2.-Select Device Manager in the new window, if you see any yellow question marks (other devices) those are drivers that need to either be updated or installed, if none are shown, check to see if you can find a listed microphone driver.
3.-go to the Acer Downloads and support page and select your laptop model
NOTE****** I've done this for you here---> ASPIRE 2920 ASPIRE 2920z
Make sure you select the correct model, as this will affect the results of the download greatly
4.-Download the appropriate drivers and restart your computer.
IF THIS DOES NOT RESOLVE THE ISSUE, CHECK YOUR MICROPHONE VOLUME, AND IF NO ISSUES CAN BE FOUND, BRING YOUR LAPTOP TO A LOCAL SERVICE TECHNICCIAN TO REPLACE THE MICROPHONE, IF LAPTOP IS STILL ON WARRANTY, TAKE IT TO THE RETAILER YOU BOUGHT IT FROM AND HAE THEM FIX IT ON WARRANTY