Musicotherapy is frequently used with children suffering from autistic disorders, given their strong interest in sound objects. We propose in this paper to study the features of this mediation from the point of view of psychoanalytical theory. In this perspective, we reveal and analyze “step by step” the trajectory and the evolution of an autistic child— Harpocrates—in a musicotherapy setting over the course of one year. It appears that the integration of maternal and paternal functions are complementary during the symbolization processes, emerging from the encounter with sound and intersubjectivity. The evolution of this child helps us to understand the symboligenic aspects of sound mediation, allowing him to explore his inner experience and gradually guide him towards the use of language.