Theaterworks is exactly right on all points.

One thing to beware is that the designer often will have, what do I call it, let's say "decor" issues with what the client really wants for audio and video, so if the designer is the one you work with, you may end up with a client who asks you why you did not do something some particular way. Or suggest an alternative that you suggested and the designer shot down. Make it clear with the designer that we all want to make the most of this situation by pleasing the client, and sometimes pure design issues and audio/video issues are at odds, so the three of you need to meet to decide how things are to be done. I have been engaged, without additional fee, by a designer, and when I accidentally met the client and described what we were doing, the client said, "F*** that! The designer has his/her head up his/her ass, and here is what I want...."