I am not exactly sure which one it is, but if I have to take a guess I would say "mus_theme_malak". I could be wrong but that is my safest bet.
If that is not the one though I would go and download a program called "Miles sound tools", it's free, and it will allow you to listen to all of the music files and voice over files in the game so all you have to do is go through each one and listen to it then you can determine what one it is.
One more option is (if know how to read kotor's syntax) you could decompile the script used in the encounter with Malak and find out which one it is from that providing that they changed the area music using it.
Any option you take though I wish you luck in and if I have time later on I will go back to that level and listen to the music so I can figure out which one it is for you.