cedar cove
Implement something like the below for when NPCs are talking so you aren't standing staring at them in silence. I don't know the name for these kinds of sound effects but animal crossing and Undertale are the first games I can think of that did it.
https://www.youtube.com/watch?v=YekLRkiIW4Y&ab_channel=Daks
https://www.youtube.com/watch?v=gGROjaIw5Xw&ab_channel=Bradly105
tawdry latch
The game already tracks "voice" as a distinct evidence point, so it feels like a natural fit. You could do it almost like the different hand writing styles where an NPC spawns in using one of several voice patterns (maybe pitch shifted randomly as well). You could then add a mission-type where you plant a wire tap without being caught in someones room, or use it to provide evidence for someone who thinks their partner is cheating.