When you change the snts and save them with different name you have to re apply the sounds to the monster inside the model editor. So here's how you would do it:
Save the edited snt file with your name. Open the brute ent file in the model editor and go to settings/animations. In there you should see all of the brutes animations and each animation has events, which is basically the sounds that play at a certain part of an animation. Find the animations and events where you want to implement your custom sounds and just replace the values with the name of your custom snt file. You can also go to settings/user defined variables and there you should be able to find the sounds that play when the monster is activated, notices, etc.
Now after you edited the monster in the model editor don't forget to save it as a different ent so it doesn't overwrite the original.
Hope this helped lad
Edit: Looks like Mudbill ninja'd me on this one lol