Where do you write for? Your own work, as a solo artist/band? Or for a soundtrack or something? This is something I've been wanting to get into and would love more information about.
It's not something you pick up just like that. I've been making music as long as I've been programming (about 27 years). So take this "passive income" and divide it by all those hours spent. You'll get something close to zero.
There are books on the topic. I recommend "Tunesmith" by Jimmy Webb. It's about the art and the industry.
On another note, it's really hard for me to bring the two things together. I could program audio software but that's programming, hardly any "art" involved. Lately, I've been making music for computer games but that's just the art, no technical knowledge required. As long as I can think, it's been either/or.
I'm actually that "rockstar programmer". ;)