For music production, there's a wider - and less organized - range of opportunities. If you want to sing on commercials, it's basically the same as above - most opportunities will be union based. For musician and artist projects, it's typically a matter of "who you know." In fact, the majority of this industry, union or not, is relationship based.
But here's a dirty little secret a session musician in Nashville told me about. He said whatever city you move to, find out who the best person is on your instrument (in this case, voice), and take lessons from them. If they're so good - and thereby, well known - they will always have more work offers than they can handle. So often times they will recommend one of their students to take the work for which he/she knows they're a good match.
On average, professional singers can expect to earn about thirty dollars per hour, or approximately $45,000-$65,000 per year. However, the bulk of a creative professional's lifetime earning potential resides in contractual payments made for ownership and licensing of creative works.