At that point, we need to define the meaning of the term Yoga as well as of the term therapy.
Yog or Jog: in Sanskrit it means to join, to put under a yoke, to unite, and by extension the various ways or techniques which help doing so, which lead to the union, union being the highest signification of the word Yoga.
      Therefore, we shall find many different ways which are conducive to this goal.

Gyâna (Jñâna) Yoga, the Yoga of self knowledge: knowing our voice to know ourselves, the voice being the reflection of our deep nature, of our psyche; working on our voice to work on ourselves.
Bhakti Yoga, the Yoga of devotion, of love: to sing Dhrupad music or any sacred music, we have to devote our time, our life to it and when at times we merge into the music, being one with the sound, with our voice, with our higher Self, then our heart opens and what we experience is pure joy, is love.
Karma Yoga, the Yoga of action for the action, not expecting any fruit or rewards of any kind from it: the spirit of Dhrupad is to sing as a prayer, as worship, as an offering or a meditation, with bliss as a goal.
Singing is thus the way and the goal.
Mantra Yoga, the Yoga of words of power (Mantra): the sacred syllables, the poems or prayers sung in Dhrupad are words of powers.
Kundalinî Yoga: in singing the scale we relate the tones to the various planes of our being regulating and balancing the energies in our physical as well as psychic bodies.
Hatha Yoga, in which one works on the physical body through Âsana (postures) and through breath control (Prânâyâma): in the ancient Indian scriptures about music it is said that the musician should sit in Padmâsana (lotus position), it involves that the musician has mastered the Âsana of Hatha Yoga to be able to sit for hours (as one sings or plays for hours in Indian music). Singing is Prânâyâma for through Alankâr (exercises on the scale), through rhythms, one will automatically and gently work on ones breath.  
Râjâ Yoga, the royal Yoga as it includes all the other forms of Yoga in it. 
 
Here it is important to note that in any form of Yoga the ethical rules, Yama and Nyama are extremely important and should not be disregarded or neglected, else the techniques could even be dangerous for the spiritual evolution, as they could inflate the ego instead of having it merge into Nâda, the sound or into Samâdhi. The Yama are the restraints and the Nyama the observances which secure a correct behaviour bringing harmony and happiness in one's life and in the world.
The Sâdhanâ or spiritual effort is necessary and the goal should not be lost!