Not sure of what your question is. There are the 3 major _applied.maths/music_ topics :
* Musical signal processing (automatic transcription, recognition).
* A.I. and machine learning (automatic improvisation, computer assisted composition)
* Physics & acoustic of musical instruments and sound synthesis.
The last one is special because the human voice is a musical instrument, and voice synthesis and analysis is a very active research field since more than 40 years.
All those subjects involve many mathematical tools : harmonic analysis (Fourier transform), differential equations & operator theory, convex optimization, probability and statistics, graph or group theory..
See this for a good start on the physics of stringed instruments.