SENSi CHiLD Calming Twin Size... https://www.amazon.com/dp/B095L6D7TG?ref=ppx_pop_mob_ap_share
He also has a glow in the dark blanket and a weighted blanket on top. This is the one we bought for him.
Telling them something is too loud does not give them a reference point to tie back to. It's not too loud to them, so they have no idea what you're talking about.
Try something like a decibel meter and tell them that their voice needs to stay under a certain level accommodating both of them as much as possible. If you can find a decibel meter app on your phone it probably has a graphical version of this although it won't be nearly as accurate and consistency will take a hit.
Weighted blanket? I have two versions of this one, with washable duvet covers: YNM weighted blanket
(I am autistic, and I provide overnight care in my home for an autistic adult with high support needs. We both sleep better and longer with these weighted blankets. She used to get up at 4:30 or 5am, but now it’s more like 7:30-9am, yay!)
A bit of Google-fu has led me to a brand called Dr. Bob's that is unflavored but still contains fluoride. I haven't tried it, so I can't attest to it, but it looks promising.
If you want to reduce noise while talking, you want stuff for competition shooting.
If you just need a bit of a reduction, I use a Bose over-ear set (AE II corded model). I've had them for coming on six years, and the electronics are still going strong. The cushions wore out, but I replaced them about two years ago with a third party product (this one: https://www.amazon.com/dp/B07N1CCC7W) which is better than the set of cushions that came with the Bose originally. They're more comfortable, have a better fit to my skull, and improve the already great sound as a result. Sometimes I just play white noise from my phone, but usually it's movie and video games soundtracks, or my 11-hour "Battle Songs" playlist when I need more motivation.
Bose is no longer the clear champion of sound, but that's because other companies have caught up, not because they're slacking. If you want to sample some, go to a Best Buy. One of these years I want to splurge on real noise-canceling, but I've got medical bills first.
Maybe you coul d consider a noise machine; it has to be a certain kind for us to work . We love this one (see below) and use it during times ike Fourth of July, when we travel at noisy hotels, thunderstorms, when we need quieter moments , etc.
Here is one we like but there are many.