Complete arcade mode with the characters listed and you will unlock the corresponding voice announcer.

Akihiko, Chie, Elizabeth, Labrys, Mitsuru, Naoto, Teddie, Yosuke, Yu, and Yukiko - each of those characters will be unlocked as a round voice announcer

Shadow Lab - Rise will be unlocked as a round voice announcer

Every single character - the "Ring Announcer" will be unlocked as a round voice announcer