Various experiments on vibrating gas columns and on frequency measurements with glasses and pipes have been presented in recent years in the "iPhysicsLabs" column. The determination of the sound velocity in different gases by measuring the sound running time has also already been proposed in an earlier paper. This article now adds another experiment to this series. Just by using a commercially available whistle (e.g., dog whistle, bobby whistle, or shrill whistle) and a smartphone with a spectroscope app, the speed of sound in various gases can be estimated. This experiment's chief merits are the straightforward experimental setup, the consumption of only small amounts of gas, the high measurement accuracy, and especially the simple theoretical background.