We propose an easy experiment that allows students to determine the speed of sound through a simple time-of-flight measurement using two smartphones. The concept of using the sensors in mobile phones for physics experiments has become a well-known option for science teachers. Since these devices are readily available to most students and teachers, experiments can be set up at little to no cost while generating fascination and motivation for the students thanks to the novelty of using their own devices in an unusual way. From all the sensors available in modern phones, the microphone offers the best temporal resolution by a huge margin as it records samples at a rate of (typically) 48 kHz. As this fast sampling rate is a requirement for high-quality audio recordings, it has been around long before smartphones offered other sensors, and audio inputs have already been used for experiments on soundcards in desktop computers.