One other option I had forgotten about is that all default sounds can be overridden, so if you only need zombie sounds and not robot sounds you could use this method. The Depot level shows how to do this, it involves creating an epd file using the episode editor that allows you to customise certain things. What I could do is add a user defined enemy type with some default sounds and then designers can override these. Not completely flexible but a lot less effort for me than the alternatives.
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