1)
Does this refer to a particular sin?
1.
Malbim: Menasheh said in his Tefilah that if Hashem will not answer him, he will know that He is like all idolatries, which do not answer prayer. He was still unsure whether or not Hashem answers Tefilah. This Safek is Aven (sin).
2)
What does Hashem not hear?
1.
Rashi: He did not do to us according to our sins. He did with us as if he did not see or hear sin in our hearts.
2.
Radak: Even if I saw in my heart evil, to do it, even though it is revealed in front of Him, as if I said it, He did not hear it. I.e. He did not consider it a sin for me. Hashem does not join a bad thought to [be considered like] deed, unless it is [denial] of Emunah in Kel.
