Let's get straight to the point.

I was rooting around the .paks in the game files and found that pakchunk2-WindowsNoEditor contains the theme music for all of the killers. I tried switching some around and repacking, but unsurprisingly, 'EAC client cannot be instantiated'. Oh well, I can still give the names of the files. Keep in mind that all of them are located in Content\WwiseAudio\Windows (I imagine that on macOS or Linux this would be different) and are in .wem format.

Generic killer theme (Trapper, Wraith, Nurse, Shape, Hag, Huntress, Cannibal, Nightmare, Pig): 763663832
The Hillbilly (CHAPTER 0 - Original Killer Trio): 937793208
The Doctor (CHAPTER IV 4 - Spark of Madness):
The Clown (CHAPTER VIII 8 - Curtain Call): 348343780
The Spirit (CHAPTER IX 9 - Shattered Bloodline): 797000496
The Legion (CHAPTER X 10 - Darkness Among Us): 367363084
The Plague (CHAPTER XI 11 - Demise of the Faithful): 144218523
The Ghost Face (CHAPTER XII 12 - Ghost Face®): 384980657
The Demogorgon (CHAPTER XIII 13 - Stranger Things): 786860721
The Oni (CHAPTER XIV 14 - Cursed Legacy): 616814446
The Deathslinger (CHAPTER XV 15 - Chains of Hate): 653097507
The Executioner (CHAPTER XVI 16 - Silent Hill): 718509398
The Blight (CHAPTER XVII 17 - Descend Beyond): 816330409
The Twins (CHAPTER XVIII 18 - A Binding of Kin): 903240932

I don't know what you'd do with this information, but it's here nonetheless.