See the diagram that is attached. When you are exactly between the speakers, the pure sound waves interfere constructively. This is shown in the top diagram: the wave from the left arrives with the same phase as the wave from the left. At the moment shown, the troughs will reinforce each other, and the two in-phase waves will continue to add together as they each move toward the center. When you step closer to one speaker, the wave from that speaker travels a shorter distance while the wave from the other travels a longer distance. The 2 waves aren't in phase, and interfere destructively. This is shown in the bottom diagram. At that point, the two out-of-phase waves will continue to cancel each other as they each move toward the center. Incidentally, if you keep on moving in the same direction, they will eventually interfere constructively again.