It's because you're using the PS2 voices and that route doesn't exist in that version (so the voiced lines don't exist). Same goes for ero content, it's not voiced/doesn't exist in the PS2 version. However, if you got both the old PC and new PS2 voice files then the "PC only" sections should just default to the old voices instead of being voiceless. There's a short guide for it here - Seiha Translations - View topic - Duel Savior Überpatch/Issue Reporting (Updated 1/16), under -Voice Actor Configuration-.
Still, it's a bit odd since if you didn't mess with the voice files at all it should play the PC voices for everything, and the Harem route is fully voiced.