slow and soft- they are two different aspects of speech. slow refers to speed while soft refers to volume here. from your post i assume you are talking about soft speech (correct me if i am wrong here).
from your other posts, i have gathered that your child is cautious, observant and introvert, once outside home. his hesitation to open up to non-family members is very much there. he might have lot of ideas, but doesnt feel comfortable in sharing them freely.
you may want to work on his faith in the outside world before working on his expressive speech. he needs to feel reassured that people outside are basically nice, that they wont hurt, dismiss or ridicule him. sometimes as parents, we caution our kids too much about safety, manners, which leads to this sort of hesitation with outsiders.
work on his self-confidence generally. do not judge him , neither negatively nor positively (believe me, positive judgment is also bad). when he makes a drawing, explain what you see in the drawing with details, instead of calling him or his painting good. when he does a good thing dont call him a good boy. instead, say what he did what a good thing. same with bad thing. notice good things as much as you notice a bad thing. generally we tell our kids when they do a bad thing(like disturbing when on phone call), but fail to tell them when they do a good thing(like not disturbing when on phone call).
also, you may use creative speech presentations at home to work on his self confidence as regards expressions. encourage him to give you mini-presentations on a given topic (bizarre ones like "what if there were chocolate rains") everyday. have others sit in when he is presenting. do not cut any idea. clap in the end. you may also have role-plays with him where you become an elephant and he is taking your interview.
have sent you a link through PT inbox. check it out