long post incoming, tldr at the end
are you thinking of doing music 1 or music 2? the courses require (in my opinion) different skills, with music 2 demanding a lot more with music theory. i did music 1 and my friend from another school did music 2, and for both courses there are some significant differences worth considering.
i believe both courses require a core performance, core composition and of course the aural skills written exams + your chosen electives (which includes the option of one or more viva voces). i was able to perform and compose for pop songs, theatre songs, etc (contemporary music) whereas my friend in music 2 had to perform and compose to more traditional styles of music eg. baroque. and as wollongong said earlier music 2 students are required to be able to sight read and notate unfamiliar music on the spot in the aural exam; in music 1 these skills are not assessed.
in my opinion, music 1 is great for contemporary singers, guitarists, bassists, and drummers in particular whereas music 2 might be more suited for those who play band/orchestral instruments or are traditional singers (though there are no restrictions on your instrument either way, we had several cello players in our music 1 class). piano players i would say could fit well into either course in the performance regard.
i’m aware that some schools only offer one of the two music courses, let alone music extension. from my experience, i did five major works and found music to be the one with the least demanding coursework. pick electives that suit your strengths (i did 3 elective performances) and as long as you practice with your music tutor at least once a week for the whole year you will be fine. to me, aural skills is not that difficult and there are hundreds of past paper questions you can do + the concepts are relatively simple as long as you can use the right terminology. i didnt do music as a class at all in year 9/10 and i was able to pick up the content of the course in year 11 just fine. the prelim course is very similar to the hsc course, with the only notable difference for me was that a viva voce task was required in year 11 whereas it was not in year 12 unless you choose that as your elective.
my opinion: do it! music is a fantastic course option for musicians of intermediate/advanced skill levels but be prepared for both performance and theory components. if you love music and are willing to commit time to preparing performances at a high quality and studying the theory, you will be fine. if you have any questions i am happy to help!!
tldr; decide which course (music 1 or 2) will be more suited for you, as music 2 is harder in terms of the skills you will need as a musician. i recommend music 1 for contemporary musicians and music 2 for traditional musicians, though this is only a suggestion based on my experiences. the coursework is not heavy as long as you commit in every class and have a music tutor to practice with every week. there are mandatory components of performance, composition and theory/analysis but if you arent the strongest in one or two of these areas the prelim course will teach you so dont worry too much. best of luck with your subject selection <3