Sound emissions of laminar premixed flames stabilized on the outlet plane of a burner located at one end of a rectangular duct were experimentally investigated. Specifically, this research applied the approach of direct acoustic admittance measurement to investigate and elucidate the close relations between sound emission, non-reactive burner acoustic admittance, flame equivalence ratio and combustor length. The admittance data were obtained by directly measuring the amplitudes of velocity and pressure oscillations, and the phase relations between the oscillatory velocity and pressure. Results of this research reveal that flames would hardly induce sound emissions without the “right” acoustic property of the non-reactive burner system. Also, the reactive burner acoustic admittance is dominated by the non-reactive burner acoustic admittance. The flame is important in determining the strength of possible sound emissions. This study also shows that the fundamental frequency of the combustor system should be within the range of critical frequencies of the non-reactive burner system to sustain possible sound emissions. The critical frequencies, the frequencies for possible sound emissions and the “right” acoustic property of the burner system could be approximately predicted by knowing the distribution of the imaginary parts of non-reactive burner acoustic admittance.