The most interesting basement I have ever built is a ten by twelve room that most people would walk past without noticing.
It is a recording studio. About $80,000. And almost all of that money went into things you cannot see once it is finished.
A room inside a room
You do not soundproof a room by putting foam on the walls. That is the thing most people get wrong. Foam changes how a room sounds inside itself. It does very little to stop sound from leaving.
To actually stop sound you have to build a second room that does not touch the first one. Separate framing, separate surfaces, breaks everywhere the two would otherwise connect. Every place the inner room touches the outer room is a path for sound, so the job becomes an exercise in not touching anything.
That is why the square footage is small and the number is not.
The doors weigh 300 pounds
Doors are where most soundproofing falls apart. A standard interior door is hollow and there is a gap under it, and sound walks right through both.
These doors weigh about three hundred pounds each. They have a mechanical seal that drops down when the door closes, so when it is shut there is no gap anywhere around it. Hanging something that heavy so it swings true and seals every time is its own problem.
Air is the hard part
Here is what people do not think about. A sealed room does not breathe.
You cannot just cut a vent, because a vent is a hole and a hole carries sound. We ran ventilation through a rivet box with an inline fan so air moves without giving sound a straight path out.
Same thinking on the electrical. Everything is surface-mounted and sealed with putty, because a normal outlet box is a cavity cut into your wall, and a cavity in a soundproof wall is a leak.
Even the lights. The recessed lights are mounted entirely within the room rather than cut up into the ceiling above, so the ceiling stays intact. They change color, which was the fun part after all the serious parts.
What I took from it
I have done a lot of basements. Bars, bathrooms, rental units, home theaters. This one taught me more than any of them, because every single decision came back to the same question: does this create a path?
That question turns out to be useful everywhere. It is the same question behind a shower that does not leak and a bathroom fan that actually vents.
The homeowner records in there now. One-room podcast and music setup, and you can stand outside the door while it is running and hear almost nothing.
If you have got a basement and something unusual in mind, I would rather hear the strange idea than the safe one. Those are the jobs I remember.