Didn't bother reading the other responses so if this has already been said I apologize but take my word for it because I am currently a student studying physical therapy.

The short answer is.. do not work out sore muscles. Contrary to popular belief your muscles grow outside of the gym. The soreness you experience is the result of tearing the muscle fibers of a particular group of muscles. Once torn, or post work out, your muscles begin to repair and build themselves back up stronger. If you work out muscles that are already torn you are not giving yourself enough time to repair. This soreness, however, does not prohibit you from working out a different group of muscles. This fact justifies the work out "splits" that many exercise enthusiast plan their work outs around. Basically, you split your weekly workouts into different body parts for different days leaving adequate repair time. I personally work back/chest one day, biceps/triceps another day, and shoulders/traps/legs to finish my split.

Okay I'm done.