I'm not looking for the dusty old cliche "he does through his word".
While it may be a cliche, it is also true. For most things, God speaks to us through our study and meditation upon the scriptures.
What made the prophets and apostles so special that God spoke to them audibly and no one else?
There are quite a few assumptions wrapped up in this question. Let me straighten this out:
1.) Prophets and apostles were/are not "special" except that God has called them into faithful witness.
2.) God speaks to people other than prophets and apostles.
3.) "Audibly" = capable of being heard; loud enough to be heard; actually heard.
4.) People can "hear" God without God making pressure waves in the atmosphere that are registered in the human ear.
5.) I have heard testimony of "audible voices" (pressure waves registering in the ear) from credible people, but that actually opens the door to the idea that people could be pranking/deceiving the hearer. I have not knowingly heard an audible voice in those terms.
6.) I hear the voice of God from time to time in my thoughts - thoughts that are distinct from mine, and much more intelligent and perceptive - when I am seeking God's guidance, but more often when there is a practical or spiritual human need where I am called to intervene. I have learned to discern it more easily over the years and it has never been wrong.
7.) Exactly once, I heard from God in an extended dream/vision during an extremely difficult time in my life. God made three unlikely promises to me in that time and two were fulfilled within the year (by circumstances outside of my control), and I've watched the progress of the third one taking place over the last 11 years in a situation completely out of my hands.