The lines, "[a]lack, my child is dead, And with my child my joys are buried!" can easily be found Act 4, Scene 5. Lord Capulet speaks these lines when he beholds his daughter has died during the night before her wedding day, not knowing that a potion is actually allowing her to fake her own death. His audience of these lines is a few different things. Just like Nurse and Lady Capulet do in lines just above these, he is addressing his lamentations to himself, to the air, to fate in general, and to any one who is also sharing in his grief. We can tell that his lines are partially being addressed to fate because in the lines just above those in question he personifies time and asks it, "Uncomfortable time, why cam'st thou now/ To murder, murder our solemnity?" (IV.v.63-64). In other words, he is rhetorically asking time or fate why it has such poor timing that it should ruin such a festive occasion as Juliet's wedding day. The next couple lines are clearly addressed to the deceased Juliet because he uses the word "thou," meaning you(it), as we see in his lines, "O child! O child! my soul, and not my child! / Dead art thou" (65-66). Hence, the last couple of lines in his lamentation speech are addressed to pretty much anyone who will hear him, be it anyone who is in the room such as his wife, Nurse, Friar Laurence, Paris, the deceased Juliet herself, or even again the personified time or fate.
In terms of meaning, these lines are obviously lamenting that Juliet has died. But more importantly, we must also remember that Juliet was his last living child left, making her an only child. As an only child, she in line to inherit Lord Capulet's legacy. Hence, his "joys" partially refer to the fact that he no longer has an heir. But "joys" also refer to any joys a parent feels from having a child.