You really shouldn't worry about sounding like an idiot in class because I doubt it has a strong correlation with grades. I pretty much stopped going/paying much attention to one of my classes spring semester because the prof was completely incompetent and I somehow ended up with an H in the class despite the fact that I most definitely was not one of the "smartest" in the subject. Really.
I haven't heard of anybody failing a class or leaving for academic reasons, though I suppose it might exist. Some people incorrectly sometimes think others are leaving for academic reasons when there are other personal reasons (I know someone who left after finishing in the top 1/3, for instance). Can't say I ever felt like the slowest/most incompetent person in class. I don't think that feeling really means much, though.
I'm actually going to show my true gunner colors and say that it can be a good idea to talk in class. Not because of grades, but for references and recs. Obviously, don't be the annoying person who talks all the time, but both employers and *especially clerkships* would like to see that at least one prof can vouch for you. It doesn't do much good to have an H in a class if the prof doesn't remember you, and in a class of 80+ students, it's easy to be forgotten. Even with a DS...if the prof doesn't have much to say besides "this person wrote a great exam" (which is obvious, because you got a DS), then that's not so helpful. I actually decided to ask a prof for a reference in a class I got a P in--and the prof, who was happy to be a rec, noted that he/she was surprised I didn't do well on the exam, because my comments in class were so good, but figured that I'd eventually get the hang of how to write exams. The prof thought the comments in class were my 'true' ability, so I'm glad I took the time to make them. Don't stress if you say something stupid, though, you'll have many more classes to impress profs in.