For a music school like berklee or any music school for that matter, the biggest factor in admission is going to be the audition, how well you sing or play is going to determine admissions. In terms of grades and test scores, a lot of the conservatories don’t even reuqire the SAT/ACT (with Berklee, it is optional), and grades are generally used as a negative (ie if someone’s grades are terrible, it can keep them out), in some cases strong grades and test scores might push a borderline candidate over the line to being admitted (My S has several friends who go to Berklee and I know people who have gone there, so I know a little bit about it). Put it this way, great SAT/ACT scores, a 4.0 GPA and AP classes won’t get you into Berklee, that I can guarantee you, if you are a marginal musician that won’t get you in.
I wouldn’t be concerned about the EC’s with Berklee, I doubt again they will be impressed by that. Where things like grades and test scores matter could be if you decide to go to a music school in a university, like U Mich, Northwestern, Rice, USC and so forth, you have to be admitted academically to get into the school as well as pass the audition, and while they give lattitude to music students (it is easier to get an academic admit as a music performance student then a straight academic admit at a lot of these schools), you still need good grades and such, plus those kind of schools also may offer academic merit aid, which is based on grades and test scores, so it is definitely wise to try and keep up your grades and get strong scores on the tests, in case you don’t go to Berklee. Not to mention you are still a Freshman, and you may change your mind about music, so having strong test scores and grades is especially not a bad idea.
One thing I would tell you, is that if you are seriously pursuing music, that if you have to choose between taking honors or AP classes and having time to practice and so forth, to give weight to the music. No matter where you go for music, if you don’t play/sing well enough, you won’t get in, no matter how good your academic credentials. As far as the EC’s go, I would give them the least amount of weight, if there are things you want to do, do them, but don’t play that game, especially if it will interfere with your music study.