I don't know about impactful paths related to art and music. Have you considered asking 80,000 hours for a free careers advising call? https://80000hours.org/speak-with-us/

There's also some useful content on the 80,000 hours website on impactful careers in arts: https://80000hours.org/topic/careers/other-careers/art-entertainment/ 

From my perspective, I wouldn't worry too much about what A-levels you choose at this stage. You're not going to pigeonhole yourself by just choosing what you find interesting and are likely to succeed in - maybe try and pick a range of humanities and sciences to cover your bases. 

I'd say if you're really unsure then try and pick something broad like international politics (or geography; where you could specialise into x-risks, AI safety, climate change or any other global problem), physics (which gives you broad science/maths skills you can apply to a range of cause areas), maths, statistics, or philosophy (which definitely has broad applications, but maybe more debatable transferrable skills). This applies to doing a degree too, and what A levels you did won't matter very much after you do the degree. 

A couple of scattered thoughts: 

• The career advisor/ university advisor I had at school was very unambitious, and it's really cool that you're thinking in this way! However, as others have said, don't worry too much about A Level choice. Your choice of A levels won't define your life.
• I don't think it's a particularly strong sign of anything that you aren't passionate about a particular path now. I only found out that I deeply enjoyed and cared about philosophy after I left school - many great subjects aren't taught in exciting ways.
• I'd probably suggest keeping numeracy (via maths, or failing that, a science or economics) as part of your roster if you can - from my and my friends' experiences, it does seem to be one of the doors that you can actually close at the A level stage. (I'm actually considering doing a Maths A Level or something equivalent at age 25 because I think numeracy is an area I'm particularly bottlenecked in because of my humanities-focused education).
• Good luck on your GCSEs! 

I believe that you should create your passion rather than find it. As 80,000 Hours claimed and here also. Plus, I want to say that I'm just like you! Even I'm starting my A-Levels journey this year and am not sure what field I want to pursue.

I hope you find your dream and passion career where you can have the highest impact!