I’m not sure if it works that way.
It’s possible the portal is only accessible if you walk into it from a specific direction.
Being inside the closet would make the portal impossible to reach.
Especially, as I speculate, the interior of the closet isn’t there anymore.
Well, technically, it’s still there, but anyone inside it at the time would be wondering why (from their perspective) the rest of the world just magically disappeared!
And if they spent long enough inside a closet they were incapable of leaving, they’d end up quite traumatized by the time the portal was deactivated!
What they are looking at, I cannot say, but either they’re looking at nothing or an energy barrier stopping them from leaving.
The latter one is the most likely, since if we were to apply multispatial physics, it would be very very bad if they were physically occupying the same space-time co-ordinates as the monster going through the door.
Basically. there would be a small biomolecular explosion as their cells merged and neither of them would survive!
And the portal would be destroyed in the process, as the energy would feedback into it.
I assume the monsters do know where each door goes, as they open specific doors for one reason or another.
How they target their spatial co-ordinates is unknown, other then picking X door to Y location.
I’m assuming that the doors were manufactured long ago and either they don’t know how they work or that job is handled by the technical/science division.
After all, you don’t need to know how a dimensional portal works to turn a handle and open a door.
But they would have to be making new doors every so often, for when new houses are created which aren’t linked into their transport network.
There are two possibilities, either they don’t know where a door is going to go until someone goes through it or they have a scanning device that locates doors to replicate them, but can’t provide any information about the residence it’s connecting to.
But it seems that they have no ability to see what is on the other side of the door, because monsters are frequently surprised by something unexpected on the other side after they go through.
There is also a scene where they open a door without the benefit of a closet connected to it, so they may not need a physical locus to create a portal.
Singing possibly, the others I doubt it.
Based on what they say, you require a certain amount of volume to harvest energy from the sound being produced.
Shouting would overcharge the system and break it, the volume is too high, generating too much energy for the system capacitators to conduct into the storage canisters.
The other sounds would be too quiet for the system to pick up, the equivalent of background energy levels.