If the battery is still connected then you can try to turn off the system, disconnect battery, make sure ac adapter is connected and then turn it on. If it remain same and if you notice the same problem with a headphone connected, then next we can try to run system file checker which basically looks for corrupt system files and performs repair as required.
A Click Windows icon (Start) -> All Programs-> Accessories. Right click on Command Prompt, select Run As Administrator. A command window should open
Type the following command and press Enter
This should start the system file checker which searches for corrupted system files and replace the same with the correct ones. Once completed, restart the computer
B Another option is testing the system in clean boot environment to eliminate possible problems due to startup programs
Click Windows icon
Type msconfig in the Search box and press Enter
Click on the Startup tab
Click Disable All
Click Apply and Close, restart the system.
C Run audio troubleshooter from the following link
Click FixIt link, click Run when prompted.
D If it does not help, right click on the small speaker icon at the lower right corner of your screen, select Playback devices
If you have more than one device listed under Playback devices, then look for the device which do not have a green tick next to it. Right click on it and select Set as default
Then right click on the device which was previously selected as default, and then set the device as default audio device, click Apply and Ok
Click Ok again
If it fails too, then I do have another suggestion to confirm if the problem is due to corrupt drivers/OS files Or the problem is infact with hardware (audio chip). If you like to to try that, then here are the steps
Create a bootable disc of Ubuntu (another operating system) . We can try to start the system from a Ubuntu disc. It will allow you to run the operating system from the CD, you do not need to install Ubuntu on the hard drive. Download ISO image file for Ubuntu
(download the file named ubuntu-11.10-alternate-i386.iso)
Download the ISO image file and burn it to a CD. Make sure that the image file is properly burned. This guide will show the procedure
Now use this CD to boot the computer. You may need to press F12 during startup to get a boot device selection menu and then select your CD drive and press Enter. Once it starts from the CD select the option "Try Ubuntu". It should now start the operating system from CD without installing it.
Once Ubuntu starts, test sound. If the same problem occur when in Ubuntu then it will confirm that the problem is with hardware.
If the problem do not occur while in Ubuntu, then the problem is with Windows system files/drivers and we need to address that issue.
Please let me know results, I will list additional steps/options..