disclaimer: while I've only fully extracted Tsukihime, both Plus Disc and Kagetsu Tohya are built with the same tooling so everything should still apply.
Before we start you should have some installation of Tsukihime (not the remake), we're only going to use a couple files from it; but these are the actual game contents. Addtionally you want to get AETools to actually extract the archives, the program isn't really maintained anymore; but neither is Tsukihime.
Audio
By default Tsukihime streams it's audio from the CD, if you have the japanese release or an ancient translation patch you'll have to rip the audio yourself or find a source online.
The Mirror Moon translation includes a patch to the game that alters the way the game does music, you'll be able to just find the music in the CD folder or somewhere near the game's installation.
Game script
The game's script is found inside the nscript.dat file, this .dat archive contains a single file 0.txt.
This file might not display well in all applications as it is SHIFT-JIS encoded, you should be able to save a copy of the file as UTF-8 using notepad; otherwise there exist various tools to do this for you.
NScripter's language is very primitive, but Tsukihime's script is pretty hefty so I would advise against dropping it in a heavy IDE.
What is of note is that you have 2 types of sections: f-blocks dictate the flow of the script, if you are interested in all the choices the game makes you should look here, s-blocks contain the scenes, this is where the text lies alongside what graphics to show and what audio to play.
Assets
These are all the audio, backgrounds, character graphics and other tidbits the game uses, you'll find these in arc.sar.
if you have arc.nsa and arc1.nsa they aren't too relevant (mostly copies of files found in arc.sar).
PS: if you have any notes on this please let me know. Additionally if you have knowledge/experience with the remake (or mahoyo HD) I would love to hear of anything you have.