4. doors--the lips, which are closely shut together as doors, by old men in eating, for, if they did not do so, the food would drop out (Job 41:14; Ps 141:3; Mic 7:5).
      in the streets--that is, toward the street, "the outer doors" [MAURER and WEISS].
      sound of . . . grinding--The teeth being almost gone, and the lips "shut" in eating, the sound of mastication is scarcely heard.
      the bird--the cock. In the East all mostly rise with the dawn. But the old are glad to rise from their sleepless couch, or painful slumbers still earlier, namely, when the cock crows, before dawn (Job 7:4) [HOLDEN]. The least noise awakens them [WEISS].
      daughters of music--the organs that produce and that enjoy music; the voice and ear.

JFB.