Use AudioToolbox Framework and add the AudioServicesPlayAlertSound / PlaySystem Code.
If you use AudioServicesPlayAlertSound function on those devices, which doesn't support vibration, your user will hear a beep instead of vibration. The AudioServicesPlaySystemSound function does nothing on these devices. So depends on a function, you can decide, which is better for your users.
For Mac using kSystemSoundID_FlashScreen to display a flash of light on the screen, or kSystemSoundID_UserPreferredAlert to play the alert specified in the Sound preference pane.