I didn't able to hear sound in my computer from speakers and headphones.

There can be more than one reason, why the computer can’t play any sounds. These are discussed below:
If you have a desktop computer, it may or may not have a separate sound card. But in case of laptops, the sound card is integrated into the motherboard and cannot be removed, so some of the solutions may not apply to laptops. Also, new laptops come with built-in speakers, so the external speakers and cabling problems may not apply here, unless you use external speakers or headphones.
So, where do you start, when you can’t hear any sounds from your computer?
Before we start with the solutions, did you try the simplest solution first? Rebooting or restarting the computer is the simplest solution and should be tried first. If you didn’t already reboot, try it now. The sounds may just work after a reboot.
Can you hear the Windows startup sounds from the computer? If you can hear the Windows startup sounds, the sound card is working fine. If you can’t hear any startup sounds, enable the System Sounds in the Control Panel, if they are disabled. If you aren’t able to enable the system sounds and they are grayed out, you may need to install the sound driver.
Are you trying to play an audio CD, and not getting any sound? If you can hear the System Sounds, but can’t hear anything from the audio CD, even though your audio player software tells that it’s playing the disc, the problem is most likely with the digital cable that goes from the CD/DVD driver to the sound card (or to the motherboard).
One side of this cable goes into the back of the CD/DVD drive and the other goes into the sound card or on to the motherboard (in case of motherboards with built-in sound).
Are the speakers connected to the proper sound port / jack, at the back of the computer? Are they powered ON? External speakers generally have a separate power adapter, which plugs into the wall socket or any other power source. Is the speaker volume control, set to a comfortable level? If not, turn it to the middle position.
If in doubt, test your speakers by connecting them to another sound device like CD/DVD player or a radio/tape player, or any sound source. If you can hear sounds from the speakers, they are fine.
Check that the speaker pin is connected to the green jack on the back of the computer. Also, check that the sound (check speaker icon in the system tray) isn’t muted. Such a simple thing, which can happen accidentally, can confuse some people when they can’t hear sounds from their computers. This can get overlooked because the speaker icon may be lost between many other icons. Click it and un-mute the sound, if it’s muted.
Note: If you can’t see the speaker icon in the system tray, you may need to enable it from the Control Panel. From the Control Panel, double-click “Sound and Audio Devices” and check the box that says “Place volume icon in the taskbar” and click OK.
Try plugging in a pair of headphones in place of the speakers, to check whether you can hear anything. Play a song or music CD or any sound file, which you may have on the computer. If you don’t have one handy, you can run a command to test your sound:
Click the Start button, and then click Run (or Search, in Windows 7 / Vista). Type the following command and then press the Enter key: dxdiag
Click on the ‘Sound’ tab to see the details of your sound card. You can also test your sound card from this tab. Note the name of the sound card and the manufacturer, on a piece of paper.
If you can’t see the name of the sound driver, or aren’t able to hear the sound by using the ‘Test DirectSound’ button, or you get an error, the problem may be with the sound driver. If the sound worked sometime ago and now it isn’t working for some reason, the simplest solution for re-installing the sound driver is to do a System Restore. Restore the system to a date that you know, when your sound worked. Check out the following links for a short tutorial on System Restore
Note: Current documents, files and e-mail are not affected by System Restore. If you get an error like “Restoration Incomplete. Your computer cannot be restored“, undo the last action by selecting “Undo my last restoration”.
Here’s a youtube video on how to restore a system with System Restore: http://www.youtube.com/watch?v=mU5yqvye57E
Here are step-by-step tutorials on System Restore:
For Windows XP:
http://www.howtogeek.com/howto/windows/using-system-restore-in-xp/
http://www.5starsupport.com/faq/xp-system-restore.htm
For Windows 7 or Vista:
http://www.howtogeek.com/howto/windows-vista/using-windows-vista-system-restore/
http://www.vistax64.com/tutorials/76905-system-restore-how.html
For more information about System Restore, visit the following links:
http://www.microsoft.com/windowsxp/using/helpandsupport/learnmore/systemrestore.mspx
http://support.microsoft.com/kb/306084http://en.wikipedia.org/wiki/System_Restore
http://windows.microsoft.com/en-US/windows-vista/System-Restore-frequently-asked-questions
If System Restore didn’t help or you get a message that System Restore couldn’t be completed, try the following:
Start the Device Manager (Start -> Run -> devmgmt.msc and hit the Enter key). In Device Manager, Click the + sign next to “Sound, video and game controllers”. See that the sound driver that you noted from dxdiag above, is listed.
Doube-click this sound driver. Device Manager should give a status message that the driver is working fine. If the driver isn’t there or if it shows a yellow exclamation mark, shows under other devices, or gives any other kind of status message, reinstall the driver. In any case, it doesn’t harm to uninstall and reinstall the driver.
From this sound driver properties page, you may also click the Troubleshoot button, to open the Sound Troubleshooter in the Windows Help and Support Center.


You can also visit the following link about tips for fixing common sound problems:
http://windows.microsoft.com/en-US/windows-vista/Tips-for-fixing-common-sound-problems
Hope that helps,
Regards,
Raspal

For Windows XP Driver Files and How-to Resources, visit XPDrivers.