Yes, as long as time is spent outside of school on the activity. This is a great thing to put on your college application- it shows devotion to other organizations.
I would say being in school band is exactly an extra-curricular high school activity. Anything that shows you have school-related interests outside of classes is a plus!
Colleges obviously look at bands as an extra curricular high school activity because they not only want a bunch of nerds. Music is the biggest interest of most people.
Yes. Band involvement is an indicator of academic ability, especially mathematics, and a strong work ethic. Band shows commitment, practice, and willingness to work with others and take direction. Yes!