The article presents a new method to measure heat release rate disturbances from flames when optical access is limited. The technique is based on the determination of the travel time of ultrasonic waves propagating through the flow. The link between heat release rate and sound travel time disturbances depends on the configuration considered. An expression is established here for the case of unconfined premixed flames driven by buoyancy forces associated with the Kelvin-Helmholtz instability formed by the interaction between accelerating hot burned gases and cold ambient air at rest. The system and the principle used for the determination of the sound travel time are then presented and validated under nonreacting conditions. Effects of the main parameters on the precision of this detection technique are examined experimentally. Measurements in reacting conditions are compared to heat release rate data obtained with another technique based on the chemiluminescence emission. A good agreement is obtained between both signals for the different cases explored demonstrating the sensitivity of the proposed technique.