If you have experience opening consoles up then go watch a youtube disassembly video of the 2ds and see if the slider is broken. If not then look in your area for console repairs, usually small mom and pop gaming stores do repairs. I would strongly vote against opening it up if you never have opened up a handheld. I have and I have repaired a few yet I still tore the ribbon cable of my ds l I was trying to do a housing swap on.
Only repair this if its 100% needed, I see you have other 3ds so I would just leave the 2ds alone if you don't have the experience.
You could also try a cellphone repair shop to take apart the 2ds to see what's the problem with the slider.
What would be the reason of not just using headphones when you don't want people to hear, it's not like full volume is actually loud lol, imo it's pretty quite. I know it's annoying having your electronic broken but if it was me id frget about it. Atleast your volume is stuck on full and not stuck off.