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Open the Realtek sound manager to see if it detects your headphones, check the volume controls there.
If not, try another headphone port - there should be one front and back.
Next try uninstalling the driver in Device Manager, restart the computer to let it reinstall driver.
Next write down the exact make/model Realtek sound device, then uninstall Realtek Sound driver software at Control Panel>Programs and Features. Now check repeatedly for Windows Updates to see if it has a specific driver it wants you to use, install it.
If Windows Update does not deliver a driver within 24 hours, import the driver from the Support Downloads webpage for your model computer or sound device.