People like it when you compliment them. So next time you are in a voice chat and your roommate is nearby, start talking about how handsome they are and how large their genitals are. They wont be bothered by your talking ever again.
Get some sort of speaker or use a smartphone. Play some music or video and put your volume around your talking volume, or a little louder. Then leave the room and close the door. This way you can get a sense of how much noise bleeds outside to your roommate.
Get a cruise ship horn (make sure it's rated for 140-150 decibels) and play it for the entirety of the call. This will make it so your roommates can't hear you speak on the call.
Since y'all aren't sharing a room, closing your door and speaking softly is usually sufficient. Make sure you have a mic that rests in front of your mouth, and isn't setting far away on your desk. That way it can pick you up easily.
If you want to reduce how audible you are outside your room you can add some sound dampening material to your walls and door and seal air cracks around your door. Those black spiked foam wall panels or heavy curtains are probably best for sound dampening, but something as simple and cheap as paper egg cartons on your walls will greatly reduce how much sound gets through.
So, there's a difference between sound absorption and soundproofing. Curtains and foam panels can do wonders for making your room sound better acoustically but won't do much to lower the amount of sound getting out. The only way to actually soundproof is either by adding a bunch of mass (bricks/concrete blocks/mass loaded vinyl) or by creating air gaps (or some combination of the two). Conflating soundproofing and sound absorption is very common but they are different. . Paper egg cartons will do little to nothing for either sound absorption or soundproofing; this myth comes from old style sound absorption foam tiles that kinda looked like egg cartons and were therefore referred to as such.
You're absolutely right about sealing door gaps helping though.
Discord has individual volume adjustor per person in chat. With my friends turning me up and me turning my mic volume setting in Windows all the way up, I'm able to talk quietly enough in the next room without bothering my wife if she's sleeping. We also have a white noise sound machine we run at nigh in our bedroom, so I'm sure that helps. White noise is probably your best bet if you're in a smaller living area.