I know this is an ancient thread, but I was trying to figure out how to turn off the narrator opening audio and kept coming back here. So this is for anyone trying to do the same thing who likewise ends up in this thread.
The file is Assets/Sounds/XML/DawnOfMan_SpeechAudio2DScripts.xml. Edit, scroll down to your favorite civ of choice, and then set iMaxVolume and iMinVolume to 0. Done, silenced.
For civs introduced by DLCs, the file is located in Assets/DLC/[DLC name]/Sounds/XML.
If I knew more about packaging Civ5 mod packages, I'd make up a quick one. But this kills the Morgan Sheppard opening narration. The MP3 still loads and plays, only muted.. I've tried eliminating the SoundID XML setting to prevent the sound file from loading, but that seems to have no effect on game load time.