I really like this game so far, but in all the conversations no voice acting shows up, am I supposed to not hear anything, or is there something I can do about it?

Most of the dialogue in the game has no voice acting. With major characters often their first bit of dialogue will be voiced so you can imagine what their voice sounds like for the rest of the dialogue, and then after that it's just a few key lines that will occasionally be voiced. Almost no minor characters have any voice acting.

The main reason for this is because PS:T has a ton of dialogue, and at the time it was released just storing that much audio on the game media simply wasn't feasible (and even today paying voice actors for that much dialogue still wouldn't be something most game devs would be willing to do).