Or maybe hes just one of those silent types

I believe he has Selective Mutism. It's an Anxiety Disorder in which a person who is normally capable of speech is unable to speak in given situations or specific people. It co-exists with severe Shyness or Social Anxiety.

good comments from both of you. I wish I had feedback from the folks that said he was born mute....

i think the captain's vocal cord got sliced or shot when fighting girlycard and walter

I think its because he don't like to talk, is a man of few words or maybe he made a silence vote.

i think it's because he's the strong/ tough silent type

maybe girlycard ripped his tongue out or something, i dont know.

I get the impression he's not 100% willing in his association with Millenium. Perhaps he's silent because they either a) have direct control over his ability to speak b) have sworn him to silence with some sort of threat or c) he doesn't believe in the cause anymore and has no reason to threaten or grandstand.