I bought an N3DS xl saturday , it is used but it is in perfect state of conservation. But I found the sound of it kind of weird compared to my O3DS xl, it's louder and the sound is clean, but the sound seems to be slightly muffled, its normal?
Have you tried enabling "Stereo" sound on system settings? "Surround" is enabled by default and it doesn't sound too well compared to "Stereo" imo.
Fun fact: I also bought a used New3ds today (Sadly, 11.4...) and the sound was the first thing I worried until I remembered that I needed to enable "Stereo".
