The previous Pasuk implies that the voices ceased before the hail and rain. Why does it say that Pharaoh saw that the rain, hail and voices ceased?
Riva: When Pharaoh saw rain with the hail, he saw that Hashem has mercy even amidst His anger, so he said "Hashem ha'Tzadik." He asked only for the voices and hail to cease. 1
Malbim: Really, first the voices ceased to be produced above, but what was produced above was heard moments later on the ground. The hail and rain ceased after the voices, but what was falling did not reach the ground, so it seemed that they ceased first. This gave room for Pharaoh to err 2 and say that it was not like Moshe said.
Moshe was surprised that also the rain ceased, and Hashem told him that it was to induce Pharaoh to harden his heart, in order to send him more plagues.
Alshich: Pharaoh assumed that once Moshe prayed, there would be no new hail, but any hail already in the air would fall, or melt and fall as rain. He saw that the moment Moshe lifted his hands, no hail or rain fell. He concluded that it already stopped even before Moshe prayed. (Even though it says that Moshe left Pharaoh and the city to Daven (Pasuk 33), perhaps Pharaoh sent a Shali'ach to see if it stopped right after he Davened - PF.)
In the warning about the Makah and the discussion of prayer to remove it, there was no mention of rain. Why does it say that the rain did not reach the ground?
Midrash Seichel Tov: [Even] rain that comes from melting hail did not reach the ground. 1
Ba'al ha'Turim: This teaches that Tefilah of Tzadikim withholds rain, and [we learn from David ha'Melech that it] brings rain.
Oznayim la'Torah: Hashem stopped the rain from falling, even though neither did Pharaoh ask nor did Moshe Daven for it to stop, in order to trick Pharaoh into believing that it was not the work of Hakadosh-Baruch-Hu, therebycausing him to harden his heart and change his mind about sending Yisrael out. 2
Perhaps this was so Pharaoh could not point at a drop of rain and say 'it was a tiny hailstone that later melted!" Similarly, Moshe said 'about midnight' to thwart any claim that the plague was not like Moshe said. This is unlike the Malbim (refer to 9:34:151:2). Even if before, Hashem gave Pharaoh room to err (the magicians were able to imitate the first two plagues; also refer to 9:7:151:1), perhaps this was only while Pharaoh had free choice, but not once Hashem hardened his heart, starting with the plague of boils. (PF).
Here it is implied that Pharaoh hardened his heart himself. Why, later in 10:1, does Hashem say "I hardened his heart"?
Moshav Zekenim: The latter Pasuk reveals that really, Hashem hardened his heart. Pharaoh hardened his own heart only after the first five plagues.
Riva: Hashem stopped the rain in order to help Pharaoh harden his heart. (PF) Refer to 9:34:151:1.
Why does it say specifically here that Pharaoh increased to sin?
Moshav Zekenim #1: Because Pharaoh saw that they needed rain and it did not fall to the ground. He increased to sin due to his frustration.
Moshav Zekenim #2: Moshe had not said that the rain will cease, therefore Pharaoh concluded that he is a liar.
Rashbam: After having admitted "va'Ani ve'Ami ha'Resha'im" and continuing to detain Yisrael, he is called Meizid.