This may be happening for one of these reasons:

• Check the volume level using the Volume Control applet. To open it, double-click the speaker icon, which is usually located in the bottom-right corner of your screen.
• Check all the connections on your computer, especially those to the microphone and speakers, to ensure components are properly connected.
• Confirm your system has the drivers installed for the sound card. Refer to the documentation provided by the manufacturer or the system reseller.
• If you cannot get your sound card to work properly, check with your system administrator or contact the manufacturer's technical support.