In this paper, some requirements of user interfaces for musical activities are investigated and discussed, particularly focusing on the necessary distinction between interfaces for musical activities and interfaces for musicians. We also discuss the interactive and cooperative aspects of music creation activities in CODES, a Web-based environment for cooperative music prototyping, designed mainly for novices in music. Aspects related to interaction flexibility and usability are presented, as well as features to support manipulation of complex musical information, cooperative activities and group awareness, which allow users to understand the actions and decisions of all group members cooperating and sharing a music prototype.