Midrash Rabba Shemot parasha 10:
וַתַּעַל הַצְּפַרְדֵּעַ וַתְּכַס אֶת וגו' תני רבי עקיבא אומר צפרדע אחת היתה והיא
השריצה ומלאה את ארץ מצרים אמר לו רבי אלעזר בן עזריה עקיבא מה לך אצל הגדה כלה מדברותיך ולך אצל נגעים ואהלות צפרדע אחת היתה ושרקה להן ובאו.
Shemot 7:27-28וְאִם-מָאֵן אַתָּה, לְשַׁלֵּחַ: הִנֵּה אָנֹכִי, נֹגֵף אֶת-כָּל-גְּבוּלְךָ--בַּצְפַרְדְּעִים."And if thou refuse to let them go, behold, I will smite all thy borders with frogs.
וְשָׁרַץ הַיְאֹר, צְפַרְדְּעִים, וְעָלוּ וּבָאוּ בְּבֵיתֶךָ, וּבַחֲדַר מִשְׁכָּבְךָ וְעַל-מִטָּתֶךָ; וּבְבֵית עֲבָדֶיךָ וּבְעַמֶּךָ, וּבְתַנּוּרֶיךָ וּבְמִשְׁאֲרוֹתֶיךָ
And the river shall swarm with frogs, which shall go up and come into thy house, and into thy bed-chamber, and upon thy bed, and into the house of thy servants, and upon thy people, and into thine ovens, and into thy kneading-troughs."
Shemot 8:2-4:וַיֵּט אַהֲרֹן אֶת-יָדוֹ, עַל מֵימֵי מִצְרָיִם; וַתַּעַל, הַצְּפַרְדֵּעַ, וַתְּכַס, אֶת-אֶרֶץ מִצְרָיִם."And Aaron stretched out his hand over the waters of Egypt; and the frogs came up, and covered the land of Egypt.
וַיַּעֲשׂוּ-כֵן הַחַרְטֻמִּים, בְּלָטֵיהֶם; וַיַּעֲלוּ אֶת-הַצְפַרְדְּעִים, עַל-אֶרֶץ מִצְרָיִם.
וַיִּקְרָא פַרְעֹה לְמֹשֶׁה וּלְאַהֲרֹן, וַיֹּאמֶר הַעְתִּירוּ אֶל-יְהוָה, וְיָסֵר הַצְפַרְדְּעִים, מִמֶּנִּי וּמֵעַמִּי; וַאֲשַׁלְּחָה, אֶת-הָעָם, וְיִזְבְּחוּ, ה.
And the magicians did in like manner with their secret arts, and brought up frogs upon the land of Egypt.
Then Pharaoh called for Moses and Aaron, and said: 'Entreat the LORD, that He take away the frogs from me, and from my people; and I will let the people go, that they may sacrifice unto the LORD.'"
'And the frog[s] came up, and (it) covered, etc'. A brayta (See Sanhedrin page 67): R Akiva says, a single frog there was and she multiplied and filled the land of Egypt.
Rabbi Elazar ben Azarya said to him, Akiva, what have you to do with Haggada? End your words and go to (the Laws) of (the impurities) of Skin Ailments and Tents. (Rather,) there was a single frog and she whistled to them and they came.
The impetus for the midrash is that Moshe promises many frogs, but in Shemot 8:2 we read that הַצְּפַרְדֵּעַ, the frog, came up, וַתְּכַס, and she covered the land. Then we read of many frogs. A pshat explanation is that הַצְּפַרְדֵּעַ is a collective noun, referring to a group in the singular. R Akiva gives one midrashic explanation how one frog became many, and R Elazar ben Azarya offers another. What, though, is the basis for the whistling frog?
Well, the verb is שרקה, whistled. I would claim there is a drash from Sharatz, with the emphatic Tzadi switching with the emphatic Kuf. R Akiva says צפרדע אחת היתה והיא השריצה, there was one frog and she hishritza, multiplied. R Elazar ben Azarya says צפרדע אחת היתה ושרקה להן ובאו, there was one frog and she sharqa, whisted. This would be based on the earlier pasuk, in which Moshe promised: וְשָׁרַץ הַיְאֹר, צְפַרְדְּעִים, וְעָלוּ וּבָאוּ בְּבֵיתֶךָ.
Another midrash claims that all the water in Egypt turned to blood - even the spittle of the Egyptians. The source of this is Shemot 7:19:
אֶרֶץ מִצְרַיִם as if it were רק מצריים, the spittle of the Egyptians. This is another tzadi - quf switchoff.