In Schaefferian theory the term sound object refers to every sound phenomenon and event perceived as a whole, as a coherent entity and heard by means of reduced listening which targets it for itself, independently of its origin or its meaning.
The sound object is defined as the correlate of reduced listening: it does not exist "in itself" but by means of a specific foundational intention. It is a sound unit perceived in its material, its inherent texture, its own qualities and perceptual dimensions. On the other hand, it represents a global perception, which remains identical through different hearings; an organised unit which can be compared to a "gestalt" in the meaning of the psychology of form.
Schaeffer suggests that there is some confusion concerning the notion whilst adding: a) The sound object is not the sound body, b) The sound object is not the physical signal, c) The sound object is not a recorded fragment, d) The sound object is not a notated symbol on a score, e) The sound object is not a state of mind (it remains the same across different listening modes). (Paraphrase of Michel Chion (1983). Guide des Objets Sonores. Eds. Buchet/Chastel, Paris. 1995 translation by John Dack/Christine North.)
UNESCO DigiArts- Introduction à l’histoire et à l’esthétique des musiques électroacoustiques: Session 3 - Du sonore à l’artistique: le timbre musical reconsidéré ( French )