Yes, you sound different when you speak than when you write. The carefully massaged sentence, paragraph, or turn of phrase is unlikely to come out of your mouth in the same way that it reads on paper. Writing, reading, and unscripted speaking are all different mental processes.
You can create a consistency between your spoken style and your written style, to be sure. When you are creating something, though, it’s usually easier to do the bulk of the work in the same medium (and yes, speaking is a medium) that your audience will receive it. You’ll certainly write out large portions of your talk as you’re creating it, but you’ll need to do equal amounts of verbal, spoken creating to really get the tone down properly. You’ll do a bit of reading from your notes when you are practicing, but you can’t just practice by reading from a written script if you want to give a notes-free, natural sounding speech.
Dancer Aziza teaches her students that how you practice is how you perform. Are you practicing in the same medium in which you’ll be performing?