Windows 8.1 Pro,High Definition Audio Device is installed and work.But,when I plug in headphones in front panel,speakers in back panel stopped.Till now they working in same time together.Why?

More often than not when a device isn't working it is because the driver has become corrupted. Re-installing a fresh copy should help



http://windows.microsoft.com/en-us/windows7/Update-a-driver-for-hardware-that-isnt-working-properly?SignedIn=1

http://captaindbg.com/how-to-update-drivers-on-windows-7/   (works on win 8 & win 10)