Generally, we use "so" to say something positive. "She's so pretty." "He's so smart." But we use "too" to say something negative. "He's too fat." "He's speaking too quietly for me to hear."

too...............for so................that He's speaking too quietly for me to hear him He's speaking so quietly (that) I can't hear him The coffee is too hot for me to drink The coffee is so hot (that) I can't drink it The concert finishes too late for me to go The concert finishes so late (that) I can't go

"so quietly" is more of a meaning of "very quietly" and more of a compliment. When you use "too quietly" it means too low for you to hear. e.g. He is too quiet ( or too noisy) for my taste. It means you do not like this particular trait.

You could, but you'd have to rework the rest of it to be good English: e.g. "He’s speaking so quietly that I can't hear him".