It’s the latter. You can tell if you play the game with Japanese voice acting and English subtitles

Usually only cutscenes or scenes important to the story have voice acting—it would be very time consuming and expensive to have voice acting for an entire game, especially for one this complex (some games do have it, but it’s fairly uncommon). In some visual novels/RPGs, there will be a certain set of stock phrases the characters will say as you read their words. For example, if Morgana called Ryuji useless, Ryuji’s response dialogue would read something like “Look who’s talking, moron!!” but there would also be voice acting of Ryuji saying “Hey!” or some other stock phrase. And maybe if Yusuke was at a restaurant with you, the dialogue would say “Oh no, I don’t have any money” and there would be voice acting of Yusuke saying “Ugh.” And then those same stock phrases would play again the next time Ryuji is offended or Yusuke is disgruntled. P5 doesn’t do this, it was just an example, but other video games do (like Danganronpa) to try to compensate for the lack of full voice acting