Seeing as you were ranked 1st internally, you will get the highest HSC external mark as your internal. This is averaged with your own HSC external mark and you'll get your overall HSC subject mark.
The HSC subject mark is then scaled based on the strength of your subject (Music 2's)) cohort. Music 2 has pretty good scaling, but no it does not usually scale better than all sciences. Don't talk about scaling unless you actually know what you're on about.... Oh and when i mean sciences scale better, i mean sciences like physics scale better than music to for the same aligned mark, not the scaled mark for the percentile.
As you said you believe you did extremely well in your actual HSC exam, you'll most likely end up with a very good overall subject mark and so scaling will be very good anyway