Will I get banned for changing voice files?

I've been using it for close to a year and nothing close to being warn or ban. But it is kind of a grey area because you are "changing files" but then again the files are still there it's just replacing what is current (usually NA voices) with what ever region voice you want to use.

It would be nice if Riot made some way to officially do it, but I doubt they'd do anything about it.

Technically you're not allowed to modify the game in any shape, way, or form. I will add however that Riot will never actually audit your games code to my knowledge, and even if they did, so long as your modifications aren't providing an advantage over other players, you're not likely to get banned for it.

I read a statement from riot somehwere that custom skins, and map skins, and voice files / announcer packs are OK because they do not give you an inherent advantage over your opponents.

Zoom hacking, FOW hacking, and scripting however, are no bueno

Nope, you are completely fine to change them.