Default system voice is done by Mizuhashi Kaori (Len). To unlock extra system voices done by other voice actors (selectable in the Special menu), repeatedly clear the Survival mode (meaning win 22 matches and go through the entire roster of playable characters) with Hisui & Kohaku.
The following is a list of unlockable characters and features. Complete the game once to unlock Special mode on the main menu, from which you can review any CGs, endings, and voices you unlock through the methods below.