Who called amidst affliction?
Rashi: All of you called amidst the burden of serving Egypt, "and I released you." Radak - it says "va'Ta'al Shav'asam El ha'Elokim Min ha'Avodah", "va'Yishma Elokim Es Na'akasam" (Shemos 2:23-24).
Malbim: Also now that we were afflicted with hunger 1 , Hashem answered us covertly and took us out to freedom from the affliction on this day. This is like he took out Yosef, and decreed [now] about satiation after the hunger, like He decreed in the days of Yosef.
Malbim (1): It was a time of famine, and on Rosh Hashanah they prayed for rain, and they were answered.
Why does it say "E'encha b'Seser Ra'am"?
Rashi: You called covertly, between Me and You, and I answered with the voice of thunder. I publicly did mighty and awesome acts.
Radak #1: I hid from you the fury with which I struck Egypt.
Radak #2: This refers to Keri'as Yam Suf -] "va'Yashkef Hashem El Machaneh Egypt b'Amud Esh v'Anan" (Shemos 14:24). It says about this "Kol Ra'amcha ba'Galgal" (above, 77:19).
Malbim: In the days of Yosef, He answered covertly, for openly it seemed that Pharaoh answers. His answer was hidden under the clamor of the officers and Teru'ah of Pharaoh and his words. Now, Hashem's salvation is revealed via thunder that came with the rain. Hidden under the thunder is Hashem's answer to the Tefilah.
Why does it say "Evchancha Al Mei Merivah Selah"?
Rashi (from Mechilta, Masechta deb'Chodesh 1): Even though it is known and tested in front of Me that later you will rebel against me at Mei Merivah (now I did wonders to save You).
Radak: I did much good for you - I released you from Avodas Egypt, I passed You through the sea, I tested you and made you thirsty, to see if you will stand [properly] with Me in a test, and you did not stand. You quarreled with Me in Refidim about the water.
Malbim: The rain that descended now is called Mei Merivah, for it is water about which Bnei Yisrael quarreled with Hashem. Via withholding rain, He tested whether they will heed His voice.
