Let's suppose that your friend talks in a violent tone. His way of talking is intimidating and that makes you uncomfortable. You want him to talk more softly.
How can you tell him that politely?
How about "Could you speak a little more softly?"
"Softly" sounds more like volume. I suggest you use "gently."
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.