It seems that VOICES are coming in a soon future in Nickelodeon All-Star brawl. (and some others features).

　• Latest free DLC character will be coming to the game pretty soon
　• A bunch of characters and stages in the works
　• Crossplay will be available on Q1 2022
　• We’ve HEARD your requests, you’ll be HEARING from us soon 😎— Thaddeus Crews (@Repiteo) December 16, 2021

Voice Acting confirmed for Nickelodeon All-Stars Brawl

‘A bunch of characters’ currently in development for Nickelodeon All-Star Brawl, developers hint at addition of voice acting

Nickelodeon All-Star Brawl Appears Set To Add Crossplay and Voice Acting In Upcoming Updates – IGN

Probably thanks to the incoming release of the game: Multiversus .. with voives inside! But it’s a start.