The speed of sound in air mainly depends on temperature and flow properties along the propagation path of acoustic signals. In turn, distributions of these quantities can be determined from sound speed measurements along different propagation paths. In this article, a modular measurement system is presented which is suited to demonstrate the effects of surrounding conditions on the speed of sound, e.g. for educational purposes, and which can be used for an easy testing of new hardware components or algorithms. Underlying mathematical relations are explained and uncertainties are discussed. A sample application of the system within a laboratory shows the effect of local heating on the sound speed along several propagation paths which differ in their spatial distribution with respect to the heating source.