Sorry if someone's already done this, but I made a list of new music tracks and voice changes I found while scoping out all of the Smash Ultimate stuff. I listened through the Direct, the website, the individual character videos, and every gameplay video I could find, and this is what I got.
Brand New Music Tracks
Smash Ultimate Main Theme
Vega Stage remix (Street Fighter 2)
Brinstar Depths remix (Metroid)
Main Theme remix (Breath of the Wild)
Splattack! (Splatoon)
Delfino Plaza remix (Super Mario Sunshine)
Jump Up, Super Star (Super Mario Odyssey)
Luigi’s Mansion remix (Luigi’s Mansion)
Bowser Battle remix (Super Mario Bros. 3)
Destiny (Ablaze) remix (Fire Emblem Awakening)
Termina Field remix (Majora’s Mask)
Beyond the Distant Skies remix (Fire Emblem Binding Blade)
2:00 AM remix (Animal Crossing)
Woodman Stage remix (Mega Man 2)
Id (Purpose) remix (Fire Emblem Awakening)
Duck Hunt Medley remix (Duck Hunt)
Lost in Thoughts All Alone or End of All remix (Fire Emblem Fates)
Splattack! remix (Splatoon)
Vs. Ridley remix (Metroid series)
The first 4 tracks listed are on the Smash Ultimate website, the next 2 are are in the direct, and the rest are in the individual character videos. There are also new tracks in the individual character videos for Daisy, Diddy Kong, Olimar, Toon Link, and Wii Fit Trainer that I had trouble identifying, so if anyone knows what tracks they are, feel free to say so.
Regarding voice acting, my discoveries are more limited, but I do have some things. Mario, Ness, Pit, Sonic, Cloud, Bayonetta, and both Corrins have the same voice clips as they did in Smash 4. Ice Climbers have the same voice clips as they did in Brawl. Bowser has animalistic voice clips again, but I couldn’t tell if they were recycled from a previous game. Inklings have the same voices as they do in Splatoon and Splatoon 2.
Yuri Lowenthal reprises his role as the English voice for Marth, as does Greg Chun for Ike. David Hayter reprises his role as Snake. Kengo Takanashi reprises his role as Link. I’m almost certain that Takashi Nagasako came back to voice Ganondorf again. Zelda has a new voice actress, but I don’t know who it is. Sheik and Fox have new voice clips too, but I’m pretty sure it’s still Jun Mizusawa and Mike West.
And that’s all I have. Again, if you have anything else, feel free to add to it.