The previous verse connotes 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; he said "Hashem ha'Tzadik." He requested only for the voices and hail to cease. Moshe was surprised that also the rain ceased. Hashem said, it is so he will harden his heart, so I can bring three more plagues.
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 come to the ground, so it seemed that they ceased first. This was room for Pharaoh to err 1 and say that it was not like Moshe said.
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 pray (verse 33), perhaps Pharaoh sent a Shali'ach to see if it stopped right after he prayed - 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 Sechel 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 that it] brings rain.
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 connotes that Pharaoh hardened his heart himself. Below (10:1), Hashem says "I hardened his heart"!
Moshav Zekenim: The latter verse reveals that really, Hashem hardened his heart. Pharaoh hardened his own heart only in 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): He saw that they needed 1 rain, and it did not fall to the ground. He increased to sin due to his pain over this.
Moshav Zekenim (#2): Moshe had not said that the rain will cease, therefore Pharaoh concluded that he is a liar.
Rashbam: After he admitted "va'Ani v'Ami ha'Resha'im" (and returned to detain Yisrael), he is called Mezid.
The text of Moshav Zekenim says 'did not need', but it seems that this is a printing mistake. (PF)