In the short story "The kid that nobody could handle", the story ends with this advice from Helmholtz: "Love yourself and make your instrument sing about it." In what sense does his advice apply to all people, not just musicians?

Your instrument, outside of music, is your own voice. If you love yourself and tell the world (using your 'instrument'), then you can feel fulfilled. This advice is good for anyone who feels like they aren't heard or seen, they can use their voice to speak up and create change for themselves and others.