Physical inactivity is a worldwide issue causing a variety of health problems. Exploring novel ways to encourage people to engage in physical activity is a topic at the forefront of research for countless stakeholders. Based upon a review of the literature, a pilot study, and exit interviews, we propose an app prototype that utilizes music tempo manipulation to guide users into a target heart rate zone during an exercise session. A study was conducted with 26 participants in a fifteen-minute cycling session using different sonification mappings and combinations of audiovisual feedback based on the user's current heart rate. Results suggest manipulating the playback speed of music in real time based on heart rate zone departures can be an effective motivational tool for increasing or decreasing activity levels of the listener. Participants vastly preferred prescriptive sonifications mappings over descriptive mappings, due to people's natural inclination to follow the tempo of music.