So here's in short what I attempted to do:

1. I added a new line of dialogue in my mod.

2. I took an existing voice file (MP3) from the vanilla game (Shivering Isles, to be exact);

3. I edited it in Audacity to remove a part of it;

4. I exported the edited file as MP3 and put it in my mod's voice folder under a proper name;

5. I generated a LIP file for it.

When I try it out in the game, the lip movement works fine, but the voice is silent.

Why?