Self-regulation strategies and behaviors are important aspects of instrumental music learning because they allow students to set learning goals by testing and controlling their cognition, motivation, behaviors, and emotions. This work investigates the self-regulation processes of four young instrumentalists (aged 10-11 years) in their initial stages of viola and violin learning, during practice with Plectrus, a real-time instrumental intonation training and assessment software. The qualitative-hermeneutical nature of this research employed a multiple case study design to investigate the construct of self-regulation within a software-supported instrumental learning process. Data were collected from the participants' practice diaries over a 4-week period. The final practice session was also analyzed from audio-visual recordings. The results indicate that, despite their limited experience, the students showed a diversity of strategies and behaviors with which they self-regulated their cognitions, motivations, behaviors, and emotions. However, not all the students employed the same processes, and there was variability in the frequency of their use. One of the students showed more self-regulatory processes than the rest and achieved the best scores, although it has not been possible to establish a relationship between the scores and self-regulation.