Don't know which zombie (therefor script) you are working with? that should be in the script attached to the AI model.
Have you changed the "properties" sounds to see if that satisfies your idea? When you click the model on the map... in that menu you will see properties, and at the bottom are sounds. I see that AI like soldiers do not have the sounds slots filled but -- the random zombie I loaded to look at had sounds assigned to the sounds slots (this was the party zombie)-- so could you try that first and let us know...
If you have a different system, it will require a closer inspection of the script.
