Implement custom voiceset: `.tlk SoundResRef`

I’m trying to implement a custom voiceset. It actually seems to be working (Yay !): I have sound playing and text appearing over characters head.

One question I have is that the .tlk file documentation includes a SoundResRef to an associated .wav file. I currently have that SoundResRef set to null, and it is still working. It’s getting the SoundResRef from the .ssf file.

When is the .tlk file SoundResRef used vs the .ssf SoundResRef? What would happen if I had it set in both places (pointing to different SoundResRef's for example)? When should I use one or the other?

Bioware file formats found at:
https://nwn.wiki/pages/viewpage.action?pageId=327727