Other answers have described how longer vocal cords will vibrate at lower frequencies. This shows that a longer cord would mean your giant speaks with a deep, booming, lower frequency voice.
But that fails to take into account that your vocal cord isn't just lengthening. It is simultaneously thickening. If your giant doesn't adapt, but just "expands" in three dimensions while maintaining the same proportions as a normal human, I'd wonder if the cords would vibrate at all. Or would they be too thick to get vibrations and just be unmoving flesh?
The other issue your giant would face is that making human vocal sounds isn't just vibration of the cords. As others have stated, there is a vast, complex set of motions involved to modulate the tones. This involves rapid changes to the jaw, tongue, etc. Your giant would have to move the tongue far faster than a human to make the same speech patterns, given that each moving part would have to travel much farther to make the shape changes.
So your giant, if it could make sounds at all, would probably talk much slower, since it's mouth simply can't re-shape itself fast enough to match human vocal patterns.
If it evolved this way from some pre-historic proto-human, then it's language would be so radically different from human language, we might not even realize it was a language and not just some weird, random, slowly changing sounds. If it was a human that magically, suddenly, grew to massive proportions, it would struggle to even begin to talk and would probably take a long time between the growth phase and re-mastering how to use it's new mouth and voice correctly.