The study investigated the impact of intervention on the students' (N=30) development of voice in speaking and singing on a short music course, which was included in the classroom teacher (elementary school teacher) education program in Finland. The second task was to create, by means of theory-guided analysis, an evaluation matrix and form to measure the development of voice, which serves as a data collection method. The areas of vocal instrument under evaluation were into body posture, relaxation, breathing, starting a sound, articulation, resonance and voice registers. These areas were measured on a 5-point scale before and after the course. Teacher as a researcher made the observation. The data were statistically processed, where p-values of the measured areas were below (0.05). Intervention helped to direct the change in a more correct and conscious direction. Most development occurred in the speech area, where 70% of students improved their performance. In singing, the direction of change was visible to 50% of the students, but a third (33.3%) remained below a good level. According to the results, more voice practice would have been needed to develop singing skills and for those students who had problems with their voice. As a further procedure to develop education, adding more guidance and follow-up to students' practice of voice usage is proposed. With the evaluation matrix the teacher was able to provide more individualized feedback on the development of voice usage than before the study.