Following the previous episode concerning the angels, who is speaking to whom here?
Rashi and Ramban #1: Hashem is speaking to Avraham - carrying out his decision to reveal the impending destruction of S'dom to him.
Ramban #2 (accordidng to Kabalah): Hashem was saying it to Himself, as He 'descended' from Midas Rachamim to Midas ha'Din. 1
Seforno: Hashem is speaking to Avraham, only whereas the earlier appearance was a mere revelation, now it was in the form of a prophecy.
Targum Yonasan: Hashem is speaking here to the angels. 4
Since they did not deserve mercy.
However the Ramban states that the commentaries agree unanimously that He is speaking to Avraham, as Rashi explains.
What is the translation of "Za'akas" and "ha'ke'Za'akasah" in the nex Pasuk?
Rashi (Ibid.): Because a 'Zayin' and a 'Tzadei' are interchangeable.
Which cries is Hashem referring to?
Targum Yonasan: It is referring to the cries of the victims of the Sodomites who robbed the poor and of those who were burned to death for frding the poor.
These cities saw how Avraham miraculously defeated the four kings. How could they do such terrible Aveiros?
Lev Eliyahu (Bereishis p.44): They were used to living based on their Midos and lusts, and did not follow intellect.