Bhashya
श्रोत्रमेव ऋक् मन साम, श्रोत्रस्याधिष्ठातृत्वान्मनस सामत्वम् ॥
In plain words
The ear is the ṛk and the mind is the sāman. Because the mind presides over the ear, it holds the nature of the sāman.
Close to the Sanskrit
The ear indeed is ṛk, the mind is sāman, because of the mind's being the controller of the ear, there is the sāman-ness.