Hey!
I'm having the exact same problem. I have a DSiXL and I can't adjust the volume at all. It's stuck at the highest and it's very annoying.
I hope someone can help! Thanks.
btw i feel this a good time to remind you guys that sea of thieves was made by rare, who also made DKC1-3 and DK64, alongside banjo kazooie and conker's bad fur day