You should choose the last subject based on your personal interests in addition to your performance potential, which are aspects that may be linked given that enjoying a particular subject could increase your chances of performing favourably in that subject. Favourable performance is also an essential consideration since you wish to study medicine, an admission requirement of which is to achieve a high ATAR. Because of this, you should ask yourself the following questions:
- Which of Economics, Music 2 or Physics are you most interested in taking?
- Which of Economics, Music 2 or Physics do you believe you can perform well in?
My understanding is that taking Chemistry and Biology is recommended for the purpose of studying medicine, so it is good that they are already part of your subject selection. An important thing to consider if you wish to take Physics is the fact that you will be taking three Science subjects, which may impose a significant workload. Depending on your ability to manage an increasing workload, this may be problematic. Unless you are interested in Physics and/or strongly believe you can perform well in this subject, I personally don't see a reason to take it, especially when you are already taking Chemistry and Biology, which in your case are more important.
This leaves you with Economics and Music 2. Here, you can recall your experience of taking Commerce and/or Music up to year 10 (if applicable) and determine whether you found any of them interesting and whether favourable results were achievable. I believe that Economics has the advantage of superior scaling, which is a secondary consideration that is relevant to a student who needs to achieve a high ATAR. However, Economics may be a slightly more difficult subject, although several students have definitely enjoyed it.
On the other hand, I think that the decision to take Music 2 would suggest that you are more interested in Music 2 (for example, you may be passionate about music) than Economics and that you are more likely to perform well in this subject compared to Economics, which in turn negates the potentially inferior scaling.
I hope this helps!