Given my relationship with sound design (as both profession and obsession), this is likely to be an overly long-winded answer to a simple question. I apologize in advance ops:...
Obviously any outdoor speakers in your cue line area should be directed away from that first room you're trying to keep silent... But as for the crowd noises outside, that's a different story...
There are two ways to isolate sound: mass and space. "Mass" is the more commonly known method to non-audio pros, and includes "sound-proofing" materials like acoustic tiles, insulation, even egg crates, mattresses, etc. Different materials have different sound deadening ratings, in relation to how effective they are in stopping airborne sound. Materials are rated by STC (sound transmission class). The higher the rating, the more airborne sound they dampen. For example:
-Hollow core door or single pane glass-----STC 20
-Single row 2 x 4 wood studs with single layer 5/8-inch Gypsum board (sheetrock) each side--------------STC 35
-Filling cavity with 3.5 inches fiberglass-------STC 38
-Single layer of 1/2" drywall, glued to 8" dense concrete block wall, both sides painted------STC 54
So of course creating thicker, more dense walls will help with sound isolation, as will replacing a hollow core door with a solid wood door. But this is only half of the picture...
The other way to isolate sound is "space". When engineers are building recording studios, they often try to construct a "floating room", in an attempt to achieve superior sound isolation. The idea is to build a room which transmits as little structurally transmitted sounds as possible. This is accomplished by having building a "room within a room", providing a gap of air surrounding as much of the room as possible. If you're very obsessive about sound isolation, you can learn to build your own floating room here:
If you want to hear a cool example of structurally transmitted sound, take an electric guitar out to your car. Get into the driver's seat, and close the door. Gently press the top of the guitar's headstock to the glass window, and begin to play the guitar. You will hear your guitar seemingly "amplified" by the car window. Many surfaces transmit sound waves well, including concrete, wood, glass, etc. Trying to use materials to block these structural vibrations will not be as effective as simply providing an air barrier (a gap between materials, that stops the structural transmission of sound).
On a practical level, and with regards to the question at hand, the most effective way to provide sound isolation between the noisy outdoor cue line area of a haunt, and the desired "dead quiet" area for the beginning of the haunt, is to provide an extra room of some kind between the two (space), and to have the main wall/door separating that room from the outside include a solid door and solid walls (mass). Since it is likely that your parking area and your haunt do not share the same floor (your cue line is likely on a paved parking lot or a grass area, whereas your haunt most likely has a wooden floor), you probably don't need to worry about sound being structurally transmitted through the floor from outside. So you don't need a studio quality "floating room". but having a small room (or even a double doorway---airlock style) between the outside and the start of your haunt should provide a decent amount of sound isolation in many cases.
If you can't achieve enough sound isolation through the use of mass and space, and your cue line noise is still bleeding into your first room (that you want quiet), you could always play "white noise" in your first room. Perhaps not quite as effective as silence, but it should help drown out the outside noise, and provide a large audio contrast for the start of your haunt. If you don't like the white noise, you could also substitute a long soft wind sound effect loop. It will also help drown out the outside noise, while changing the mood dramatically.
If you're trying to provide a stark audio contrast between a noisy cue line area and the start of the haunt, the obvious thing to avoid would be kicking off your haunt immediately with loud sound effects or music. Save those for a minute or two after your patrons enter your haunt... For when your they have adjusted to the silence (or quiet)... THEN hit them with the loud fx/music! We've probably all been to haunts that were filled with loud noise/music from beginning to end. Not a very effective sound design. Good sound design relies on contrast (just like good music). There can be no contrast if your haunt has no areas of quiet (or silence).
There are some other tools to utilize regarding contrast in sound design (such as EQ, reverb, etc.), but maybe I'll write something about those at another time, as my post is already three times longer than anyone would want to read!