I used to have a hp pavilion dv5 and i had this same issue. All it took was alittle of configuration with the driver/or the program that you use to chat with such as MSN messenger.

Read through the owners manual of your laptop to see if there is a key or key combination to turn on/off the microphone. If there is none or it is turned on go to your systems BIOS/Setup screen when you first start your laptop. Look through the different settings to see if there is an option to turn on/off the built in microphone. If there is one make sure it is turned on. If there is no option in your BIOs or it is turned on then make sure you have a driver for it on disk or a USB drive. Delete the device (microphone) in your device manager. Then restart your computer. If the device is plug-n-play Windows will find a new device on start up and either install the driver or prompt you for the driver. If the device is not plug-n-play load the driver and windows will find the device.