Don't think you'd have any issues with overheating in the closet, still might need to put something on the door. I have a small fold-out mattress that I can throw over my door for improved sound quality, something like that with some additional blankets could work out.

Good Luck!

line the walls with egg cartons

If not for fake sound reduction, at least the increase in design factor and feng shui.

does that actually work?

Depends on the room

It may not make your teammates happy or whatever, but having a (kinda loud) fan will help with cooling for the closet idea and also provide some white noise so you don't seem as loud to your parents.

You could also get a nice cardioid mic and talk a bit quieter. I use a blue yeti and it can picks up words that i mouth into it.

Doing that can reject insurance claims if your house catches fire.

Thanks for the advice :D I think I will run a fan, I do now and because of where it is placed I don't hear anyone complain about it (I place it so it all blows under my desk to keep my legs cool)
I've also invested some time and... "money" (not keygens I promise) into programs to reduce background noise that seem to help

soundproofing is more for studio/recording purposes, it just seems like you have really thin walls. soundproofing foam/solution wouldn't magically drown out you talking. it will reduce the sound by a minimal amount but i guarantee they will still know when you're awake and talking. also when you're wearing headphones and talking/comming you're a lot louder than you realize so your voice is projecting higher to compensate over the in game noise and people talking in mumble

sneak into parents room and put ear-muffs on their ears like in spongboob

Thanks for the tip! I think I will have to try lowering my volume levels for playing matches and things of that nature, and have some cheap earbuds for late night gaming :)

My computer is in the room adjoining my parents' at the moment, and I avoid disturbing them at night by using open-back headphones which give a good sense of how loud you're talking, and I use a modmic and crank the gain up so it picks up my voice well even when I'm talking very softly.

Soundproofing a room without tearing the walls out is nearly impossible.

I was wondering why you didn't comm much during that lobby.

You need something that reflects sound and something that absorbs sound. foam for instance, will absorb some sound, and if you put a sound reflecting material (like a plate of some sort) behind it any sound that bleeds through will be reflected back into the foam, if that makes sense. This is pretty much impossible to do unless you want to line your walls and ceiling with 2 inch thick foam.

also, even if you do that, it only means a certain range of frequencies will be absorbed, and some frequencies might not even be affected by it at all. also, the materials needed will probably cost you a lot of money for the amount you'll need.