The Vmo/Mmo of A340 should be 330kts/0.86
For the second part of the question, I'm not quite sure what you are asking, but the A/Thr will be engaged during descent under normal circumstances. It will either keep it in a thrust mode, which it will maintain the same thrust and change the attitude of the aircraft to get the desired speed or in Speed mode, which it will keep the flight path constant and adjust thrust for speed.
