We present a fast and robust method for approximating sound propagation in situations where audio and video frame rates may differ significantly and positions of sound sources and listeners are only known at discrete times, so that numerically stable velocities are not available. Typical applications include 3D scenes in virtual environments where positions of sources and listeners are determined in real time by user interaction. Our method employs a computationally inexpensive heuristic that converges to the exact solution for constant speeds and achieves convincing Doppler shifts in general.