The way I understand it, there are two sides to sound:
* Technical Sound Design (or Sound Engineering)
* Artistic Sound Design
Technical Sound Design has to do with the obvious -- setting up equipment, understanding acoustic concepts, and making everything sound good so the audience can understand and enjoy the performance. Generally I handle all of this, with the exception of setting the overall volume levels which will tend to vary based on the director's preference.
Artistic Sound Design has to do with picking the content and integrating it into the show. I will generally handle the technical and some artistic aspects of this, but will expect the director to be the primary supplier of source material and review anything I come up with. This also includes sound effects and voice-overs, which I can manufacture, but I will expect the director to review and approve.
Your
level of artistic involvement will depend heavily on who the director is and what their
level of talent and/or interest is on sound issues. And the more knowledge and skill you have in this area, the more they will tend to trust your judgement.
Also, sometimes the Music Director will pick the music to underscore the dialogue, and they may also be involved in making special effects. It all depends on their talent
level as well. In our non-musical productions this year there has always been a grey area between me and our resident music director as far as what we each provided for the show. But basically we both chip in, and between us, the director, and the artistic director of the theater, we have enough ideas and talent to get the show to where it needs to be. (You have to leave your egos at the door, though