In ballet, synchronizing movement with music is essential for both technical precision and expressive performance. However, younger dancers often struggle with abstract concepts such as timing due to the lack of continuous, personalized feedback in traditional instruction. This paper introduces KneeEcho Box, an interactive sonification interface for ballet training that uses real-time computer vision to track knee movements during the plié and translates them into MIDI-based xylophone tones, reminiscent of a music box. KneeEcho Box was evaluated in a between-subjects study with 22 young ballet dancers. Results show that sonification enhanced movement precision, spatiotemporal awareness, and user engagement, particularly among younger participants. These findings suggest that integrating interactive auditory feedback into ballet practice can enrich traditional instruction and open new avenues for research at the intersection of human–computer interaction and the performing arts.