The files are located in game\modules\tome-1.0.5.team, which is just a renamed zip file. However, you can change the sound effect without mucking around in there by making an addon.
Feel free to use this one as a base: (which is also just a renamed zip file)
-Extract init.lua from the linked addon and edit it with your info
-You'll want to overload the old sound file with your own. Put the replacement in overload\data\sound\talent\slime.ogg
-Zip the folder with your edited copy of init.lua and name the file tome-(your addon name).teaa
-Using this addon as-is will invalidate your save files and prevent your characters from showing up in the vault. To fix this, post
here to get uploader permission, then use the sidebar on the right of te4.org to create a new addon. You'll need the md5 hash of your addon, which you can find in te4_log.txt after starting a character with your addon. Alternatively, you can use the 1.0.6 beta, which has built-in addon publishing tools that streamlines things a LOT.