We report an experiment protocol for ultrasonic velocity measurements carried out in a synchrotron-based multi-anvil high-pressure apparatus. By adding a second reflector in the cell assembly, ultrasonic signals are significantly stabilized for experiments where samples experience structural changes such as phase transition or partial melting. Features of the elastic wave can be calculated from acoustic impedance allowing us to evaluate the possible interference between the sample reflection and other internal multiple reflections. This new protocol allows various forms of samples including cold-pressed powder samples, for which grain size and porosity can be monitored and controlled in situ during the annealing process.