। ॥ १४७ ॥
In plain words
The more the mind lets go of its objects and settles into true stillness, the more a sound — like a flute's note — is heard, growing longer and longer.
Close to the Sanskrit
As, and to the degree that, the mind attains motionlessness through the cessation of [involvement with] sense-objects, so, correspondingly, a sound, like [that] of a flute, longer and longer, is heard.