I did something similar ages ago in GG, many time doesn't work with GG but with GGMax, so, not fresh data in my mind, but iirc I wrote a script plus sprites, so, you can use the same system to make subs in your game.
https://www.youtube.com/watch?v=f4_l1C5Jih4
Since you already have the audio files and you already know the dialogues, is easiest for you to work with.
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