Question

Theory of classical music is discussed in which veda

Answer 1

Theory of classical music is discussed in Samveda in which a morning Raga , Raga Bharva has been defined.

Answer 2

Samaveda is the right answer.

The Samaveda is one of the four Vedas. The Samaveda has two parts, in the first part there is a collection of music (melody), and the second part of the Samaveda contains three verse "books".  It has fewer verses than the Rigveda.  This veda was compiled after the Rigveda.