I am trying to speak would also fit the bill as it is more often used talk.
Yes. I would in fact label it just as common as "talk" in this context. For example, if you're really irritated with somebody:
"AHEM, who told you you could speak?? I'm trying to talk here and no one is letting me!"
The boss would like to speak to you, speak slowly, I can't understand you when you speak Polish etc. I believe it's more formal than talk here.
Yes it can be used formally but also very informally:
- "Son, speak up!"
- "Hey!!! Who spoke?? I heard you.."
- "Speak here into the mic"
I hear you speaking and I concur. Talk about getting confused if you don't speak English, when you wish to talk to someone who does/