It depends. At least the Realtek HD Audio Control Panel has a tick box for muting the rear output jacks when a front phone plug is detected. Unticked, the same sound would come from both and if you wished to save your family's ears you'd simply not turn the speakers on. On my trusty old Gigabyte EP-35 the setting is hidden on the Audio I/O tab's Analog/Backplate section behind a tiny wrench symbol.