“You have to open a door or window to let the pressure out of the room.”

Who is throwing stones I'm chucking boulders😃

Overall, the room is not pressurized by your speaker.  When the cone moves forward, there is a brief increase in pressure, but when it moves backward, there is a corresponding reduction in overall pressure.  A wave has a portion that has a higher pressure and a corresponding part that has lower pressure than the ambient pressure of the room.  What you are reducing by opening a door or window is the acoustic energy in the room (less is bouncing around and more is escaping).  This will change the sound, and whether that change is good or bad depends on the particular room and system and the volume level the system is playing at.  If you are playing at such a high level that the room is overloading, backing off the volume will do more than opening a door.  In any case, just listen to the result; theory will not accurately predict the result.  

I was present when an industry expert was helping a friend set up one of his listening rooms.  He open and closed doors, moved sound absorption and scattering panels around and listened to the results rather than following any sort of formula/theory.  This room sounded better when the panels at the first reflection points on the side walls were removed, for example.  As for open doors, this room sounded better with the door to the room closed, but with a door to a closet in the room opened (the closet acted as a bass trap).

I'm not sure about sound waves in a pressurized room, and I don't listen much in closed rooms:

just to say, if you somehow let the air out, you will be making a source for sound to get both out and in. You might want some methods to curtail the sound, or direction of the sound.

Rooms and Spaces do get pressurized, and fans can only push air if they can pull the same amount, and they take the air they push/pull where it is easiest. If you have tube amps making heat, it's nice if that heat can get out, above them rather than over you, without letting sound in or out (if problematic)..

When you run your car's AC with the windows closed, at first, you might lower a back window so it can push the hot air out, then close them all.

Designing HVAC systems for Office Space, for each space, we need a changeover of ___ cubic feet/minute (heat or cooling) so a small office, we 'undercut' the door 1", that's enough. Sound getting out that slot near the floor is not too problematic in an office space, but at home, you know your volume levels ....

A larger space, a grille in the door, or return air grill in the ceiling, sized to remove the amount of air we want to push in. More air/more noise.

Executive offices, we build the walls above the ceiling all the way up to the floor above, then, to avoid telephone calls being heard in adjacent rooms, we place an acoustic boot/collar above the return air grille, acoustically lined, and you need a hole thru the wall you built above the ceiling, for the air to keep flowing. We offset the locations, so air can flow but make it harder for the sound to travel/reflect, air doesn't care about direction, just flow, sound can be diminished in/out by methods.

Some doors have acoustic drop seals, or air-flow drop seals, that lift so the door can open, and automatically drop when the door closes.

Spaces with odors, we design negative pressure rooms; spaces like clean rooms, we design positive pressure rooms, so when you open a door, air goes out or in by design, just reinforcing the fact that spaces do get pressurized, some, or more than that.

Hmm, I've always read that you should make your room as airtight as possible so the speakers or subs can pressurize the room. But this has made me curious to try listening with the door open or I also have a double-door closet in my room to try and leave open. I already have very clean and abundant bass thanks to a pair of pretty perfectly positioned (I'm trademarking triple P) Rythmik G22's, which is obviously much easier to achieve in a dedicated room.

I sometimes have to operate in a space suit in a laminar flow room.its quite uncomfortable. Maybe they'll let me bring some big speakers in and try it.just another research project🙃