I just realized you said Duck Hunt. You should rename the Duck hunt file from Wii U by adding _3DS before .nus3bank.
As for Corrin... Is the sound file named Kamui? And is the path smash/sound/vc/?
Sound effect files for characters go in smash/sound/se, voices go in smash/sound/vc. You have to locate each individual se and vc file in the pack
Ex. Melee Fox's sounds
snd_se_Fox.nus3bank goes in smash/sound/se
snd_vc_Fox.nus3bank goes in smash/sound/vc
Adding 3DS worked, thank you so much, I feel like an idiot!