I love this sort of thing, so after like a day of researching, here's what I've found on all of the fighter voices heard in Ultimate. I've split them up into four categories: Completely new voices, returning voices with new clips, fighters with recycled clips from previous games, and the new fighters. I'm pretty sure I was able to cover mostly everyone, but feel free to correct me if I got anything wrong.
Completely new voices:
- Kengo Takanashi; to obviously fit BoTW Link.
- Unknown new voice actress, but it's definitely a new voice compared to Jun Mizusawa
- Unknown new voice actress, thanks to the World of Light trailer, we know that she has an actual English-speaking voice.
- Yuri Lowethal finally brings his Marth voice to Smash; making this the first time Marth has spoken English in the series. Although I do like his take on Marth, I will forever favour Hikaru Midorikawa as the Hero King.
- Ray Chase; this is probably my favourite voice change out of all of them thus far. I've always really liked Jun fukuyama's voice for Roy, and Ray is just killing it with his spot on grunts and shouts; his normal voice is a little on the mature-sounding side, but I'm sure I'll get used to it.
- Greg Chun; I'm going to really miss Jason Adkin's dorky one-liners for Ike, but from what I've heard so far, Greg's Ike is still pretty dang good. (Even if there isn't much of a difference between his PoR and RD Ike. XD )
- Justin Anselmi; Was surprised to see that Ivysaur got a new voice. He sounds pretty dang close to Craig Blair's Brawl Ivysaur.
- Ja Green; I freaking love wolf's new voice--it fits him so well. Although I did also like Jay ward's Wolf from Brawl.
- Since they were silent in 4, their voices in ultimate are obviously new. Voice actors are currently unknown, to my knowledge.
- Although some audio clips of the Duck Hunt Dog are still from Smash 4, he now has brand new barking clips, so I'd consider it new enough.
Same voice actor from a previous smash game, but has new audio:
-Makiko Ōmoto; From what I can hear, his normal attacks use Smash 4 audio, but Kirby obviously will have new audio due to new transformations.
-Mike West; He did a MUCH better job this time around. His ADR director for 4 must have been having a rough day or something. XD
- Satomi Korogi; sounds to me like a mix of old Melee clips and either previously unused Melee clips, or brand new clips.
- Mark Lund; I thought he was pretty good in Smash 4, so I'm happy he's back.
- Fujiko Takimoto; I feel like this situation is similar to Pichu. A mix of Melee clips along with either new audio, or unused audio from Melee.
- Takashi Nagasako; fittingly reprising his role from Melee. He sounds much deeper and more sinister this time around, but that could just be due to the actor himself aging. Love his Ganondorf. (Although I'll always have a sweet spot for cranky Grandpadorf.)
- Antony Del Rio; Has a lot of the same clips taken from 4, but has a couple new ones plus actual dialogue in World of Light.
- David Hayer; Although all of his battle sounds come straight from Brawl, Hayter confirmed that he did new voice work for Snake in ultimate.
- Michele Knotz; This one I'm kind of iffy on. I haven't seen much footage of PT's voice in action, but the one clip I did hear didn't sound like it came from Brawl. I also have no idea who voices female PT, so I guess she would fall under new voices. XD
- Michele Knotz; According to the wiki, it says that Knotz's clips used in Ultimate are not ripped from Brawl. This could totally be incorrect though.
- Billy Bob Thompson; after listening to his voice clips in Ultimate, and comparing them to 4 I actually think Thompson recorded new grunts for Ultimate. I'm still hearing some 4 clips mixed in though.
Completely recycled audio:
- Charles Martinet; recycled from Brawl/4
- Reused animal noises from Brawl/4
- Reused sounds from Brawl/4
- Kazumi Totaka; recycled from Brawl/4
- Ikue Otani; recycled from Brawl/4
- Charles Martinet; recycled from Brawl/4 (Although I think there may have been new audio for him in Simon's trailer, but I don't know for sure.)
- Makiko Ōmoto; recycled from Smash 4
- Ryō Horikawa; recycled from Melee/Brawl/4
- Rachel Lillis; recycled from Melee/Brawl/4
- Samantha Kelly; recycled from Brawl/4
- Reused monster sounds from Brawl/4 (really wish they would use Kenny James in Smash, but Sakurai likes his weird creature voices. XD)
- Sanae Kobayashi; recycled from Melee/Brawl
- Charles Martinet; recycled from Mario Brawl/4 clips
- Laura Bailey; Recycled from Smash 4
- Keiji Furjiwara; Recycled from Smash 4 (I was actually surprised to find out that Mewtwo's Melee voice is from a different voice actor.)
- Recycled sounds from Brawl/4 (Not really a voice, but the beeps and boops. XD )
- Eric Newsome; Recycled from Brawl/4
- Antony Del Rio As far as I can tell, recycled from Smash 4
- Alésia Glidewell; Recycled from Brawl/4
- Charles Martinet; Recycled from Brawl/4
- Shin'ichirō Miki; Recycled from Smash 4. Funny enough, Miki also voiced Charizard in Brawl, but did completely new clips for 4.
- Reused animal sounds from Brawl/4
- Lani Minella; Recycled from Brawl/4
- Roger Craig Smith; Recycled from Smash 4/Sonic Generations, Mario & Sonic at the London 2012 Olympic Games, and Sonic & All-Stars Racing Transformed. None of Roger's Sonic in Smash is original. XD
- Masahiro Sakurai (This
still trips me out. XD ); Recycled from Brawl/4. Although his laughing audio from K.Rool's trailer might be new? Maybe?
- Hajime Wakai (Pikmin audio); recycled from Brawl/4
- Sean Schemmel; recycled from Smash 4
- Reused Robot sounds from Brawl/4
- Sachi Matsumoto; Recycled from Brawl/4
- Ummm... I guess silence recycled from Smash 4
- Same with Villager. XD
- Tania Emery (PAL); Recycled from Smash 4. This is the only voice for WFT that I could find, so I have no idea if the other regions/male voices for WFT are different.
- Kerri Kane; Recycled from Smash 4
- Kosuke Toriumi; Recycled from Smash 4
- Brandy Kopp; Recycled from Smash 4
- Sounds reused from Smash 4/Pac Man/different Namco Arcade games
- David Vincent, Lauren Landa; Recycled from Smash 4
- Adam Howden; Recycled from Smash 4, but he might have new audio in World of Light
- Caety Sagoian, Lani Minella (Larry, Morton, Wendy, and Lemmy), Mike Vaughn (Iggy and Ludwig) and Dan Falcone (Roy); Recycled from Smash 4/ Mario Spin-off titles
- Kyle Hebert; Recycled from Smash 4 (Give him his orange alt. and boom, you've got adult Gohan. Close enough, eh? XD)
- Takahiro Sakurai (Or as I like to call him, Kishibe Rohan); Recycled from Smash 4
- Cam Clarke, Marcella Lentz-Pope; Recycled from Smash 4 (I find it funny that both Cam Clarke and David Hayter are voicing in the same game once again, but Clarke isn't playing Liquid Snake.)
- Hellena Taylor; Recycled from Smash 4
New Fighters
- Yuki Tsujii; Recycled from Splatoon
- Deanna Mustard; Recycled from Mario spin-off titles
- New spooky monster sounds
- Unknown new voice actor
- David Vincent; Huh, this guy voices male Robin as well, and he's the narrator for the English dub of Jojo--as if Richter needed any more jojo vibes going for him. XD New audio
- Matthew Mercer; (Oh wow, so apparently Jojo's narrator and Jotaro Kujo are best buddies in Fire Emblem. XD) New audio
- Absolute silence
- Chunky Crocodile sounds
- I think her normal voice is generated, but in the actual game, I think she pulls a Villager and stays silent.
- Reuben Langdon; New audio
- Although I couldn't find his English voice actor, I do know that Japanese Incineroar is voiced by the late Unshō Ishizuka. I loved this man's work. Kizaru in One Piece, Mr. Satan in Dragonball and of course, old Joseph Joestar from Jojo. Man, Ultimate is just turning out to be one big Jojo reference, isn't it? Rest in peace.
So yeah, as seen above, over half the cast uses recycled voice clips. But honestly, I don't really mind it at all. Although some characters could certainly use a new recording, most of them are just fine the way they are, IMO and the time and resources spent on re-doing all of these voices could be put towards other more important things.