Another interpretation: Why did Moses say [the Song] with the word "then" (az)? With "then" (me-az) I made my complaint before the Holy One, blessed be He, as it is said, "and from then (u-me-az) that I came to Pharaoh to speak in Your name, he has dealt ill with this people" (Exodus 5:23). With the very tongue with which I sinned, with that same tongue I will set things right and praise You.
Moses Mends With the Same Tongue That Once Complained
Yalkut Shimoni on Torah 241:3
דָּבָר אַחֵר, לָמָּה בְּ'אָז' אָמַר מֹשֶׁה, בְּאָז קָרָאתִי תַּגָּר לִפְנֵי הַקָּדוֹשׁ בָּרוּךְ הוּא, שֶׁנֶּאֱמַר (שמות ה, כג) "וּמֵאָז בָּאתִי אֶל פַּרְעֹה", בַּלָּשׁוֹן שֶׁסָּרַחְתּי בּוֹ בַּלָּשׁוֹן אֲנִי מְתַקֵּן וַאֲקַלֵּס אוֹתְךָ.