The Manusmriti says:
The Ṛgveda is sacred to the gods and the Yajurveda is human; the Sāmaveda has been declared to be related to the Pitṛs; hence its sound is impure.—(124)
This is odd because Krishna says in Bhagavad Gita 10.22:
Angiras also says in his Dharma Shastra:
Aṅgiras (Caturvargacintāmani, Paribhaṣa-Kāla, Adhyāya 14).—‘In as much as sacrifices to the deities of the cremation-ground are offered with the Sāmaveda,—the sound of that Veda is unclean.’
in Smṛticandrikā (Saṃskāra, p. 160), as stating the reason for what has been declared in the preceding verse;—in Puruṣārthacintāmaṇi (p. 443);—and in Hemādri (Kāla, p. 768),—in all these to the same effect.
However, Medhatithi says:
The sound of the Sāmaveda should not he understood, by this, to be really unclean; all that is meant is that, just as Veda should not be recited in the presence of an unclean substance, so should it not he recited in the presence of the Sāma-chant; it is on the ground of this similarity to ‘unclean substances’ that the sound of Sāma has been said to be ‘unclean.’
But what is the point of comparing it to unclean things and even deliberately stating that it's impure, when you don't actually mean it is impure?