I do some local/regional jingle work, have composed music for a couple of local stage plays and for 2 independant film projects, have had songs placed in a few studio films (direct to video, no major theatrical releases....yet) and I just found out this week a song will be in a net work TV show ("My Name Is Earl").
I don't make enough to actually earn a living (music is a part time gig) but each year I make a little more than the year before. However, I commiit a lot of time and effort - if I'm not working my day job or sleeping (I rarely get more than 4 hours of sleep) I'm writing, recording or working on my chops. My goal is to retire early and have music provide part of my retirement income.
It is very hard to get your foot in the door and the competition is fierce. In my case I got a few breaks (no matter how much talent someone has, you still need a lot of luck). I was a working musican for many, many years and at one time did a lot of session work -so I established a network of contacts (recording studios, engineers, ad agency people, etc.). I sometimes get small jingle projects if some of my contacts are too busy or if the project is too small for them to commit to. I make a point of finding out who the local film people are and keep in contact with the local college (which has a "film school". In my case, I also found a publisher who focuses on source material for film and televison.
There are many books that provide a lot of good info, but at the end of the day it takes talent, hard work and the willingness to accept a lot of rejection (only a small percent of what I write actually generates any income and I get many rejections for every small success).