This paper describes the use of sound speed in flow monitoring applications in the high-pressure/high-temperature downhole environment. Downhole flow monitoring is an area that continuously receives attention for many reasons including zonal production allocation in multi-zone intelligent completions with inflow control valves (ICV), detection of production anomalies, as well as reduction of surface well tests and facilities. The propagation speed of a sound wave is a powerful tool to extract useful information from a flowing fluid medium in pipe whether the medium consists of a single-phase or multiphase flow. Considering the complex nature of the flow patterns and changing phase fractions from reservoir to surface, obtaining the propagation speed of sound in this harsh environment is not a trivial task, especially if the interest is real-time flow monitoring. The demanding applications span a wide spectrum from very noisy medium (usually created in gas/liquid flows by the presence of ICV) to very quiet medium, which usually originates from slow-moving liquid/liquid flows. Real-life examples are used for demonstrations. Although most examples are based on strain-based local sensing of the flow, the use of sound speed is independent of the methodology and can be implemented by other methods such as acoustic-based distributed sensing.