How to replace dialogue audio?

I want to change one of the files in dialog.rsd, but I can't quite figure out how.
Is there a way to do it?

Also, I think heard music modding was kind of a thing, but it was unreliable? I also kinda want more details on that.

I believe what youd do is remove the original file from dislog.rcf (its rcf not rsd) then convert the audio file you want to use into an rsd file using Lucas RSD converter, and import it into dialog.rcf. Ive never done this before so beware following my advice!

That's pretty much what I've been trying to do, but I don't know how to import it back into the RCF.

You don't. You just put it in the same directory in your mod's CustomFiles folder.

I moved it to the root of the CustomFiles folder, and it still doesn't seem to work.

I said in the same directory, meaning you follow the file structure of the RCF file in your CustomFiles folder. Like CustomFiles/conversations/whatever.rsd.

Ohhhh, I thought you meant the same directory as the rcf file. That makes much more sense.
Thanks for helping me out, I feel like a total newbie to this kind of stuff.

It's all good. Glad I helped.

To answer your question on the music side of the game, it is possible as I have tried it myself however you have to make sure that the sample rate is the same as it is in the game (22050) otherwise the track will change pitch and tempo. Another problem (only if you don't own the game on disc) is that the game will crash as it will try to find the file from the disc but because there isn't one a window will pop up saying insert hit and run disc. also because different files have audio compression your audio might come out quieter than expected. The only way I can think of resolving that right now is by turning up the volume in an audio editing software but that can make the audio sound distorted. You might need to consider this as well for custom dialogue but I haven't done that yet. the process is the same as for dialogue though so you still use the RSD converter.