oot theory time, link cant talk out loud in oot because all kokiri talk via psychic link, and only talk out loud via their fairies. oot link never learned to talk out loud because of this.
saria is absolutely psychic and we know this because you can talk to her in link's mind at any time by playing saria's song
in other games, the kokiri are koroks, suggesting that being a korok is their true form, and they don't seem to have mouths in their korok form -- if you look under the mask, altho there are holes for eyes and mouths, there's no actual hole for a mouth underneath
navi is who does the talking for link
enriches the existing story by:
explaining more of how/why link was bullied by mido and left out by the other kokiri. if link can't communicate, it's a more specific reason link was left out
explains some of the cultural importance of fairies
makes link canonically mute in OOT, which is a change i really like. it also has a different reason than his silence in pre-calamity botw, which was stress-related
questions that this theory leads to:
where are the fairies in subsequent game? this is still a question that i've had before regardless of the fairies' material usefulness for speech, but if they're how the koroks speak out loud, then it makes it confusing how they can speak out loud in WW and BOTW/TOTK -- however, given the koroks in WW and BOTW/TOTK seem to only be communicated with by some (usually children) it may be that they still communicate telepathically or by some form of magic. or, maybe fairies became a part of koroks, somehow combined with magicked wood.