This paper presents CODES – COoperative Music Prototype DESign, a Web-based environment for cooperative music prototyping. Its main goal is to allow any user – especially those with no expertise in music – to draft musical pieces collectively, in a prototyping manner. So, such musical sketches – we call them music prototypes – can be repeatedly tested, listened to, and modified, not only by their original creators but also by the online partners that will cooperate in their refinement, until their final form is reached. CODES enables sharing of knowledge by means of rich interaction and argumentation mechanisms associated to each prototype modification, which are also secure ways of providing awareness to this asynchronous collaborative environment. In this paper, we present the concept of music prototyping and introduce the main aspects related to cooperative prototyping of musical pieces, focusing on issues concerning a musical piece as a collective creation of a virtual community. We will also show some usage examples as a means to describe the overall architecture, behaviour and potentials of the CODES environment.