The production quality can range drastically. Some are only a step above your high school production, while others are springboards for shows on the way to Broadway. The majority of these jobs audition in New York City but some audition locally. And don't forget summer stock. There are theatres in almost every state.
Backstage also lists singing coaches.
Again, if you plan to dance in musicals, whether on the road or not, start vocalizing right away. If you like great pay and short spans of work, then this is the spot for you. These jobs pay extremely well but are few and far between. Even dancers on the 'A' list can average less than one week of work in a three-month period, so be prepared to have a means of supporting yourself between gigs. One way to do that is to teach at dance conventions and judge the competitions.