Hmm... That's weird. Have you downloaded the new update of the base game? There's a new update in December 15, 2024.
Sorry, I can't really help you without me being able to test it out myself. And currently, I cannot do that.
I will mention though that in the developer's page ci-en, there were two people who commented about how there were no voice in their game as well.
The first commenter first encountered this problem when he updated DLC2 to his un-updated base game. This commenter later edited his comment that it got fixed when he downloaded the newer version of the base game. However, it still was missing the voices for the DLC 1 character, to which the developers answered that they already gave the updates for DLC 1 to dlsite and FANZA, but it has not yet been updated by the respective sites.
The second commenter however confirmed that they had downloaded the new version of the base game, but for some reason, it was not 'FULLY VOICED' there. I think the commenter meant that there were no voice at all, but it's just speculation. He didn't mention, however, if he patched up DLC 2 together with it. Maybe there is a weird bug where you need to update DLC 2 with 12.13.24 version of the base game for the voice to function. I know it's weird, but it's totally possible since the first commenter did manage to play the game with voice after re-downloading the base game.