“What troubles you.” The Hebrew is an idiom, literally, “What to you?” It means, what troubles you, what is the matter, what is wrong.
“Hagar.” The angel more or less introduces himself by calling Hagar by her name and having knowledge of her trouble; things a stranger would not have known.
“voice of the boy.” Hagar was the one who was crying loudly and uncontrollably, but Ishmael also must have called out for help to God. He had been raised by Abraham for at least 16 years, and more likely 19, and he surely would have come to know, and to some extent rely on, Abraham’s God, Yahweh. The angel said to Hagar that God heard the voice of the boy, not her voice, not because God did not love Hagar, but in part to get Hagar’s focus off herself and also because God had promised that Ishmael would become a great nation (Gen. 17:20), and He would do what it takes to fulfill His promises.
“where he is.” This phrase can have a broad meaning, and likely includes both his physical location and his situation. Thus, the HCSB has, “from the place where he is,” and the NAB has, “in this plight of his.” Both meanings likely apply. God knows both where we are and our situation.