We can try to guess at what this line is supposed to mean. One of the ways someone can be left speechless is if they are stunned by something that they have just observed. If the speaker was in the middle of a speech and something very surprising happened, one way he could react is by coming to a stop in his speech. The alternative, of course, is that he goes on with his speech to try and ignore whatever was going on, hence "i'd be speechless if I wasn't already speaking." This is just a guess though, and we can't be sure without more context.

Someone is "speechless" if they are so overcome with emotion that they can't talk. Usually, that emotion is shock or surprise, sometimes sadness or anger, but if you are so emotional that you cannot think of the right words to say, you can just say "I am speechless." Or even just "Speechless." (You can also say that about someone else. e.g. "After Ted found out that Sally was really an alien from the planet Saturn here to colonize the earth, he was speechless and just stood silently for 10 minutes.") Here it sounds like the person who said that was shocked (or whatever) already in the middle of a sentence when the shock happened. Or that they're the type of person who never shuts up. They're basically saying that they're very, very shocked - but they also want to keep talking. :)

This doesn't sound like something that a person would say naturally.