Shemot 7:2 states that G-d told Moshe that Aharon would speak to Pharaoh. Rash"i further explains that first Moshe should relate to Aharon what G-d told him, then Aharon relays the message to Pharaoh.
When we look at the later verses prior to nearly each plague, G-d tells Moshe "Go to Pharaoh and say to him..." He seems to be telling Moshe to directly talk to Pharaoh, not Aharon.
Also, when Pharaoh calls Moshe and Aharon back to remove the plague, it seems like Moshe is answering Pharaoh, directly.
Another "puzzle", here - In Shemot 7:17, the speaker says, "Behold I will hit with the rod that is in my hand on the Nile waters and they will change to blood."
We see later in verse 19, that Aharon uses a rod (it's unclear if he used Moshe's rod or Aharon also had his own rod, which he used) and it is Aharon that strikes the rod, not Moshe. If Aharon was the one speaking to Pharaoh, then, what he said seems consistent, assuming that he also had his own rod, or if he was holding Moshe's rod when he spoke.
I'd like some clarification on who spoke to Pharaoh and when? Did Moshe ever speak to Pharaoh? Or was it that Aharon spoke only to tell him the message about an upcoming plague. But, to remove the plague, Moshe spoke to him?