The Key of the Ram is obtained in the Auditorium of Lahkesis after you protect the translator and "persuade" him to help you. This is a bit later in the game so ignore the doors for now.
Climb up the ladder to start dealing with the flame pillars. Use the crank you grabbed from the floor below to temporarily turn down the pillars then use the amulet to slow time. You should now be able to glide over to the platform the flames were protecting.