I have the Spanish version of FO3, and AFAIK, the English audio is not included. Is there a way to (legally) get he file and replace it? I′d rather listen to Liam Neeson and the rest of the original voices.

I think the only way to do this is to buy the English version.

So just getting different language audio files of a game I own is piracy?

Yes. The audio files are considered protected and copyrighted assets/content, and can't be legally distributed without permission.

That's a fact and seems the end of the discussion.