If the person looks uncomfortable then I kindly explain that I have a speech impediment. The way you say it evinces confidence as well. If you say, "I'm sorry, I have a speech impediment." That's fine since you're first addressing their reaction and then explaining it. Some may view this as low confidence since it can be interpreted as apologizing for it. If you say, "Oh, if you're wondering, I have a speech impediment." Pretty much the same thing. Just not including the "sorry" in their. There are various other ways, but basically just be open about it. Some people will always be unaware of such a thing. You're literally just informing them and making them aware. If you'd like a joke form to break the ice, then it depends on the conversation honestly for me. I can't give anything on the spot and I can't really think of anyway to deliver it.