The reality is that there will be kids applying who meet all of the criteria you've been hearing and believe me, it'll be immensely intimidating, but despite that, a large number of them will be rejected. Stanford could easily fill its freshman class with all of those kids who are "perfect" but that would definitely not create a diverse student body. Not to say that these kids don't deserve it or are not good enough because they are definitely some of the most qualified. However, as much as Stanford admires these kids it wants students who have a life outside of school; students who have done well academically but not for the sole purpose of getting straight A's and perfect scores but also pursuing passions and approaching things intellectually merely becuase they are curious. So now to answer your questions... I don't think you should worry about your 3 B's. From my understanding, Stanford does not look at your grades from freshmen year, although they will look at what courses you took. However, from this point on you definitely want to work hard through the remaining years of your high school and aim for straight A's. Even though several of those "perfect" kids will get rejected, their stats still remain impressive. The increasing number of nearly "perfect" students has set the standards so high that really high scores and grades are now more of a requirement than something special that distinguishes you from the crowd and guarantees acceptance. As far as weight distribution in what Stanford considers, they're all pretty important. Your grades, extracirriulars, test scores, letters of recommendation, essays, etc. I suggest looking at this link from Stanford's site:
Stanford University: Common Data Set 2008-2009
Scroll down a little and click on the link that says "First-Time, First-Year (Freshman) Admission. Then scroll down until you see a chart underneath "Admission Requirements". The chart tells you what they consider and of how much significance it has in your admission decision.
As for clubs and organizations, Stanford would rather see dedication in one or a few extracirrulars than a large laundry list of 20 activities. Although it's nice to be a well-rounded student, being involved in EVERYTHING prevents you from really being dedicated to something. I'm sure that some kids get in with having been in 20 activities, but dedication to a particular activity is so much more appealing. Also, keep in mind that that dedication definitely sparks interest in your application and can help make you stand out. Student government, being the president of nearly every club, being a member of the National Honor Society, and doing thousands of hours of community service are all admirable and definitely not something to be ignored, however I can guarantee you that most applicants will have them on their application. If you're involved in any of that then you definitely want to include it but I'm just saying that it's going to take more than involvement nearly every academic organization and club. As for what the music department is like, I'm not really sure. Sorry. I'm not a student there but I want to transfer. I applied for freshman admission and didn't get in. Since it's still the school I want to go to and I've been really persistent about it, I decided to visit the school and speak with an admissions officer to get a better understanding of what they look for and how we should approach the application process and this what she told me. I hope it helps!