The acoustic source amplitude of low-frequency modulated spark discharges is determined experimentally. Burst and pulse-density modulation are utilized in order to generate source components at frequencies much lower than the pulse repetition frequency. Pulse sequences consist of high-voltage pulses with 10 ns duration, 9 to 12 kV amplitude, and pulse repetition frequencies up to 30 kHz. The source amplitude is experimentally determined by microphone measurements in an impedance tube. Spurious components in the measured pressure signals, associated with electromagnetic noise from the high-voltage discharges, are removed by appropriate data processing. The Fourier component of the electric power at the modulation frequency is determined by phase-averaged pulse energy measurements, and the relation between electric power and sound source amplitude is revealed. The effect of pulse energy, electrode gap distance, and the number of pulses per modulation period on the initialization phase, during which HV-pulses do not generate sparks, is determined. An analytical model based on sound generation by unsteady heating is employed to estimate the acoustic source amplitude from the electrical power input; good overall agreement with the measured source amplitudes is observed. The sound generated by low-frequency modulated NRP discharges in the present work, with modulation frequencies in the range of 50-1000 Hz, can be predicted well by assuming that the entire electrical power acts as unsteady heating over the relevant timescales.