Alternate Character Voices can be unlocked via the EX Chapters in Adventure Mode, or by purchasing them from the in-game store.

Voice	How to unlock
Amitie's Alt. Voice	Purchase for 1800 pts.
Arle & Carbuncle's Alt. Voice	Purchase for 2500 pts.
Draco Centauros' Alt. Voice	Complete EX 8-5.
Ecolo's Alt. Voice	Complete EX 10-3.
Feli's Alt. Voice	Complete EX 9-7.
I's Alt. Voice	Purchase for 1800 pts.
J & L's Alt. Voice	Purchase for 2500 pts.
Klug's Alt. Voice	Complete EX 9-4.
Lemres' Alt. Voice	Complete EX 9-9.
Maguro's Alt. Voice	Purchase for 1800 pts.
O's Alt. Voice	Purchase for 1000 pts.
Raffine's Alt. Voice	Complete EX 8-9.
Ringo's Alt. Voice	Purchase for 1000 pts.
Risukuma Alt. Voice	Purchase for 1000 pts.
Rulue's Alt. Voice	Complete EX 8-1.
S's Alt. Voice	Purchase for 1800 pts.
Satan's Alt. Voice	Complete EX 10-4.
Schezo's Alt. Voice	Complete EX 9-1.
Sig's Alt. Voice	Complete EX 9-10.
Suketoudara's Alt. Voice	Purchase for 2500 pts.
T's Alt. Voice	Purchase for 1000 pts.
Witch's Alt. Voice	Complete EX 8-4.
X's Alt. Voice	Complete EX 10-1.
Z's Alt. Voice	Purchase for 2500 pts.