We study the elastic properties of two-dimensional phononic crystals made of aluminum nanocube lattices. From the experimental point of view, collective modes resulting from the coupling between cubes inside the crystal are excited and time resolved using ultrafast acoustics. We derive a general theoretical model of the collective modes. From the analytical expression we discuss the influence of numerous crystal parameters on the detected frequencies. We then present experimental results on various samples which all exhibit an excellent agreement with the model. The theoretical description reveals that collective modes propagate along the sample surface. From that we propose to use such modes for measuring the in-plane elastic properties of thin films.