This sounds like an object sanity thing. Some files may only be modified through the core directories only, the mod folder doesn't always work. So I'm not surprised to see you mention this.
I haven't tried adding .wav files and such to Elemental, so I can't confirm or deny your findings, but you might want to make a note of it in this thread:
https://forums.elementalgame.com/434780/page/1/#3378469
We probably should have a new version of that thread in the E:LH modding forum (so that people notice it), noting any changes in behavior since E:FE, but it's Heavenfall's thread so it's up to him if he wants to start an updated thread here, unless someone else feels like cutting/pasting that info into a new E:LH Mods thread.