As the question says: What should I say when I hear the Aazan (Adhaan)?
It is Sunnah to repeat the adhaan word by word when you hear the Mu'adhin's voice because the Prophet (peace be upon him) said:
When you hear the Mu'adhin, repeat what he says word for word, then invoke Allah's Blessings and Peace upon me, for whoever invokes Allah's Blessings and Peace upon me (once), Allah will send His Blessings and Peace tenfold upon them. Then ask Allah to grant me Al-Wasilah (the most superior degree in Paradise), for it is a rank in Jannah (Paradise) that is granted to only one of Allah’s Servants, and I hope I will be this one. Whoever asks Allah to grant me Al-Wasilah will deserve my Shafa`ah (intercession). [Sahih Muslim 384]