This study investigates the acoustic emissions of a water jet impinging on a flat surface of water and applies the acoustics to the study of the hydrodynamics and entrainment characteristics of the jet. As the jet penetrates the surface, air is entrained in the form of bubbles which produce sound by various mechanisms including amplification of turbulent noise, collective bubble oscillations, and single bubble resonances activated through entrainment, break-up, or coalescence. The experiment involves a fresh water jet, with a diameter of millimeter order and velocities ranging to about 10 m/s impinging from varying heights into a fresh water receiving pool in a large laboratory tank. The acoustic signal is measured with hydrophones in different positions, amplified and digitally sampled. Time-series, power spectra, and time-frequency analyses are connected to the hydrodynamic processes in the bubbly flow. Measurement and analysis of the acoustics provides a new non-invasive technique by which to establish various properties of the gas entrainment and hold-up.