I do impressions in my head of what I inwardly think they [the posters]sound like. I read all caps in a yell.
Same here, although I do occasionally sub in a "Narrator" of sorts in which I can freely choose whomever I wish. Usually the likes of Christopher Lee,Morgan Freeman, Stephen Fry, Brian Blessed, Sean Connery, etc.
It changes depending on the author, contents, and on a forum the avatar. For example, devsonfire I read in a generic male voice but every time his post says "damn it" I read it in Yasuna's (dev's avatar) voice.
First, I determine whether they be trollin', if that's the case I just read it in my voice. If it's a normal post then I assume gender, this is where I leave it if I don't think said poster is particularly active. If I know where the poster is from I form their accent in my head. If something is well written I take this as an assumption that the person has good diction.
It's like reading texts from friends; I read them in their voices.
Well, I know how Art's, Antw0n's, Jun's, and Sneaky's voice (so far, for the rest, I can't really tell who's who), so yeah.
For the rest, just generic unless they are writing it in Caps or being sarcastic.