The general answer: In general, you want vapor barriers to keep that nice, moist, warm air that's inside your home from reaching the cold outer shell of the home and condensing there, where the water begins to collect and create mold, mildew and rot problems, not to mention a severe reduction in insulation value in the wall.
A typical vapor barrier consists of 4mil clear plastic stapled directly to the studs after all the insulation and wiring and such have been installed, and right before you install the drywall.
At all penetrations, you want to do whatever you can to prevent the warm air from working its way into the penetration and into the wall cavity. With in the wall speakers, this may involve installing a baffle, or something like that. Caulking and foam insulation are good, as is expanding foam in some cases when used judiciously.
As far as the Specific answer: No one can reliably tell you what YOUR state and munincipal code requirements are going to be better than YOUR state and munincipal building regulation departments and inspectors. They are paid with your tax dollars and are supposed to be available to answer your questions. The code is supposed to represent the bare minimum that you have to do to be LEGAL, but doesn't necessarily represent the RIGHT thing to do. Check with your plan reviewer or local inspectors and have a dialogue with them.