I haven't played this game yet however I did see the video. If there was no real voice acting for the game previously, then I feel that you've succeeded with what little you had to work with. The voice audio sounds a bit grainy but it works. Did you per chance use that AI thingy to bring about a more complete Narration?
well they were a voice that did the narration before, and Dracula but I used AI to clone the voice of Alucard and Dracula from DXC because there was 5 games he did, tracked down all the audio so I could clone Dracula. the more audio you have the better the clone voice works, like in SOTN Alucard has 5 minutes of audio while everyone else has like a minute to 2 minutes, and when you try to clone them mostly sucks. even after scrapping together all the audio for Dracula it didn't clone as good as Alucard. so cloned hundreds of times and got mostly trash but got a few lines that sounded ok.
far as the old English voices I think it was more of a proof of concept that they could do an English dub. because they never used the dub but they were in the game folders, and it is the only files that are unprotected. the Japanese files are in another format so you can't play them or edit them. so my guess the English was a WIP proof of concept. something they could edit and change and they just never deleted it or converted it over to the other format.
but the English voice was ok, this is just something to chance it up, I do believe with the new sound effects for Dracula and the new voice it makes the fight better. but Death didn't have no lines that was the only lines I added to get him to talk, and translated the Japanese subtitles because the Dracula speech wasn't too good in English IMO. so my Dracula is what the Japanese script said, but change just a bit to flow better in English.
far as the voices being grainy well I made the louder to go with the music, but maybe I should had just knocked the music down. because the music seems to drowned out the voices otherwise.
anyway it's just something to liven up the game, you can watch other play throughs and see if you like the older voices better. take care and thanks for the reply.