This paper is concerned with the coupling mechanisms leading to the spontaneous generation of sound during flame propagation in a tube open at one end. We consider the cases of premixed gaseous combustion and of premixed spray combustion of decane droplets in air. The flame front propagates from the open to the closed end of a tube and, for a particular position, starts to amplify a longitudinal acoustic mode of the tube. We call this mode the primary acoustic instability and focus our study on the physical mechanisms responsible for sound amplification. Measured amplification rates are compared to calculated values. In the gaseous case, it is shown that the instability results from a coupling between the acoustic acceleration field and the geometry of the flame front separating the burnt gases from the denser unburnt mixture. The situation is quite different in the spray case. The primary acoustic instability is much stronger and results from a modification of the inner structure of the flame. This modification arises from the velocity lag of the droplets in the acoustic velocity field, leading to a modulation of the fuel flux at the flame.