Nada means voice,sound. Joining with the Universal Lord with the sound.
How to join the virata purusa, the Universal Lord, in different way.
That is called yoga.
Yoga means joining with the Supreme Lord with myself. And joinings are many ways.
NadaYoga means joining with your voice. Joining with music.
Sound is the media to join yourself with the Universal Lord is called NadaYoga.
That's why he is called NadaYogi...