He is lord of the worlds beneath that (the self in the eye), and of all the wishes of men. Therefore all who sing to the vînâ (lyre), sing him, and from him also they obtain wealth.

In plain words: This person in the eye rules over the worlds below and over all human wishes. So whenever people sing to the vina (a stringed instrument), they are really singing to him, and he is the source of the wealth they gain.