Why did Moshe think that Pharaoh would not listen to him?
(6,12) “And Moshe spoke before Hashem, saying: Behold, the children of Yisrael did not listen to me, so how will Pharaoh listen to me who has closed lips?”
The various commentaries have struggled to explain this posuk, but we can explain it with what is written in parshas Shemos, where Hashem told Moshe to go together with Aharon since he had difficulty in speaking, and also because Moshe wanted to give honour to his older brother. Moshe thought that this meant that he should speak together simultaneously with his brother, like we see from the teaching of the Mechilta on posuk 12:3 “Speak to all the congregation of Yisrael” - Moshe gave honour to Aharon, and Aharon to Moshe, and it seemed as if the speaking emerged from both of them.
But the gemora in Rosh Hashanah 27a teaches that two utterances even if they come from two men cannot be heard well, except in the case of Hallel and the Megillah whose words are dear to the congregation and so they pay particular attention and can hear even ten voices.
This is what Moshe was saying here: “Behold, the children of Yisrael did not listen to me” when Moshe spoke to them previously together with Aharon. And even though they had listened when they had addressed them on earlier occasions, and also now the words that they spoke to them should have been dear to them, nevertheless, since they were under much stress they did not hear. So if they did not listen, “how will Pharaoh listen to me who has closed lips” - and so will be speaking together with Aharon. All the more so will Pharaoh not listen since what they would be saying to him would certainly not be dear to him, and thus he will not be able to hear speech coming from both of them. Hashem then told him that he should speak first and then Aharon should speak afterwards by himself.