The helicopter was dispatched on an emerg med svc (EMS/med-evac) flt to transport a patient with subcranial bleeding back to a hospital in Winston-Salem. BFR takeoff, the plt received a wx briefing. At that time, widespread IMC prevailed throughout the area & VFR flt was not recommended. While en route, the plt climbed to 4500' MSL. In the last transmission from the acft (to hospital dispatch), the helicopter was rprtd to be aprx 12 mi from Galax (the destn). A witness in that area heard the sound of a helicopter which ceased with a 'thud' when it crashed. He looked in the direction of the noise, but he could not see anything due to fog. The wreckage was fnd aprx 10 mi SE of Galax. The helicopter had crashed just below the top of a mtn ridge at an elev of about 3500'. There was evidence the helicopter was in about a 20 deg rgt bank when it crashed. No preimpact part failure or malfunction was fnd during the investigation.