Again, thanks for your the trust you have placed in me!.
It seems to me that the correct one is 'la' because either you are talking about the voice (la voz, fem.) or the alta( also femenine) or the two words as one, (the first being a noun and the other an adjective). So , from that point of view I would say it is "la", femenine...BUT!
The 1st sentence did not sound crazy to me (at least not at first), for some reason it didn't sound 'conflictive' and slang language or coloquiall usage of words and phrases might play an important role. In other words what I am trying to say is that , if I had to shout this expression at someone in my house because they woke up a baby, without thinking of it too much, I might have use the first one...and here is why (I hope I can explain it).
The important part of the sentence for me is the part where the verb appears..."hablaste" so I am focusing on the big picture/general concept....Tú hablaste..., or "el hecho"/ the fact, In which case it would be "lo" because neuter doesn't exist in spanish, per se.
So , I if i just say " Tu hablaste muy alto ...LO cual despertó..." but If the sentence was only "Tú voz es muy alta, la cual despertó ..."
I found a very good discussion regarding this subject, and people seem to think similar to us...either they focus on the that last "alta" or on the first verb/main idea.
I hope this doesn't confuse you, it is not my idea...its actually all the way around...after thinking about it, it seems like "la" makes better sense. I know there is a rule, that has to do with this, I will try to explain it with a sentence...
Los carros y las casas son muy costosos....Carro is masculine, Casa is feminine, costoso (meaning that cost a lot) was left as masculine, because , in spanish, when talking about general stuff or neuter stuff we use the masculine form, like when we say "los niños y niñas" even some might argue equality of genders and would rather say "los niños y las niñas" but ,the thing is , spanish was fixed this way many years ago, and this is starting to change, an example I can think of is when reffering to certain professions, some years ago saying "Juez (judge)" was acceptable for both genders, "El juez Carlos..."La Juez Maria" but then , the RAE (Real academia de la lengua española) decided it was correct to use another word..Jueza, yes, with an 'a' at the end. Same for presidente and presidenta and so on.
Well, I hope this helps a bit!
Saludos desde Tijuana, Baja California, México