Infinitude wrote: there are some situations that I can do it, e.g. if the topic is something I know a lot about.
Ding ding ding! You answered your own question right here. Find groups of folks who meet up todo something you no a lot about. Join a DnD group (there are even online ones if you want to practice voice chat first before face-to-face, though I obviously recommend trying an in person campaign every once in a while till used to it if you choose DnD. I read once on these forums where someone went to cons where they dressed up, and acted as the persona. This seemed to help them act out what they normally had trouble with.
Tricks here you're going to have to learn is knowing that A) there will be times you say things that are wrong, and B) that when you gracefully admit you're wrong, people will quickly get over it. It's when things are dragged out that people feel the need to over-correct. Not saying you do drag things out (I have no idea), but I know many tend to refrain from fear of being mocked.
Personally I got yelled at a lot as a kid and teen when I spoke before being spoken to. I basically had to rewire myself to speak up now that I'm an adult so I don't miss out or have problems when I have to do things on my own in general.
Infinitude wrote:I'm wondering how most people manage to be so verbally/conversationally proficient (in my eyes) and how common it is with SA's to be mostly silent like me, and what ways there might be to try and combat this.
I have had selective mutism since I was in 3rd grade. I rarely have problems with it anymore, but it's still on occasion there. My boyfriend found out that I knock it off as soon as a subject gets brought up that I know a lot about. If I feel I have to answer questions I go from mute to eloquent immediately.