Custom player sounds
A lot of people probably know how to do this but when I wanted to do it I couldn't figure it out. Anyways, tools you need to do something like this are listed below.
Kotor Save Editor
Miles Sound Tools
Goldwave Digital Audio Editor
Now, if you've poked around the save editor before, you've probably noticed that there is an option to change soundsets, even on the notmally muted player character. If you want any of the normal soundsets listed there, you just click what you want, it's that easy. However, if you want to create a voice set for a character like Sion or Nihilus, you have to do a little more. The first thing you have to do is decide which NPC party soundset you want to overwrite. (I suggest party characters simply because they have options for opening chests, selecting them, etc.)
Overwriting sound files with a sound file is easy. Simply copy the file you want to use and rename it to the one you want to overwrite. Ex. Rename A_SITHASS_ATK to p_G0t0_Atk1.
If you want to overwrite a sound file with something from the StreamVoice folder, you have to decompress them with Miles Sound Tools first. Decompress them to a folder, and then rename and overwrite.
Then, simply go into the Save Editor and switch the soundsets. If you did it write, you should get an entirely new custom soundset.
PS- I put in goldwave in case you want to listen to the sound files before you overwrite them. I know that if you look around this is pretty easy to find, but I thought I might be able to put all this info in one place. Enjoy.