I wonder if they have looked into air curtains. Commercial installations are at a point where you have this silent, completely invisible sheet of fast-moving air that provides some degree of noise isolation as well - maybe four walls of these + a floating ceiling is enough when coupled with the noise-cancelling headphones.
It's not going to make it any worse. They just need to reduce noise levels enough to make the noise cancellation more effective, so they could keep the open booth + headphone setup instead of a soundbooth.
They also add some pink noise from the air and motors which might help.
In any case, just an idea, data on their effectiveness seems hard to find.
That would encourage a much more boring style of gameplay.
If you can’t launch a surprise attack because the audience will give it away, the best strategy would be to slowly and incrementally build up a gold and experience advantage.
A certain player who used to get accused of cheating a lot learned this trick way before everyone else, but in recent years everyone's been doing it and it can really swing games in a way that's not fun to watch as a spectator.
There was once a big StarCraft 2 tournament where one of the contestants walked out mid-event because he couldn't pull off any cheeky tricks due to the audience giving him away.
Think of it this way: any time the audience starts going wild about something one team knows about but the other doesn't, they're doing so in anticipation of dramatic moment when the other team learns what happened and must adapt while the first time is trying to exploit it.
If it's given away instead, the anticipated event never happens or is muted because the enemy is not caught off guard.
In some early tournaments, the casters would have to avoid looking at Roshan fights, because the other team would inevitably hear (or feel!) the bass-heavy sound effects from Roshan's attacks and movement.
Later versions of the client added an option to mute those sound effects, for precisely this reason.
It especially screwed over pre-siren Roshans (which don't seem to be much of a thing anymore). But yeah, there's still a lot the sounds can give away, like a Roshan play, a sneaky smoke, enemy tormentor attempts, etc.
Even without knowing much of anything about DotA 2 in particular: for any game with asymmetric information, the crowd reacting to what your opponent is doing is always going to be a major tell. At least in LoL, managing that information asymmetry is an important strategic element, can't imagine it being any less of an issue in DotA 2.
There was a recent dota2 LAN without soundproofing and the audience would ruin surprise attacks (aka smoke tanks) or surprise objective taking (aka rosh), it made the gameplay worse.
