In fact, the answer may lie in the pesukim decribing the sin, and in fact some of the standard commentaries note this. Bemidbar 12:1:
Miriam and Aaron spoke She spoke first. Therefore, Scripture mentions her first. How did she know that Moses had separated from his wife? [See below] R. Nathan says: Miriam was beside Zipporah when Moses was told that Eldad and Medad were prophesying in the camp. When Zipporah heard this, she said,“Woe to their wives if they are required to prophesy, for they will separate from their wives just my husband separated from me.” From this, Miriam knew [about it] and told Aaron. Now if Miriam, who did not intend to disparage him [Moses] was punished, all the more so someone who [intentionally] disparages his fellow. — [Tanchuma Tzav 13]But it is not just that she is mentioned first. The verb וַתְּדַבֵּר is feminine sigular, rather than masculine (=neutral) plural. Perhaps this follows from Miriam being mentioned first. But it also raises the possibility that it was only Miriam who spoke.
Indeed, speaking is something that involves two people as well as the subject matter to be discussed. The subject matter was negative statements about Moshe. (As Ibn Ezra demonstrates, the root dbr followed by a b introduction as in בְּמֹשֶׁה seems to connote negative speech.) Perhaps it would be awkwards to state vatedaber Miryam el Aharon beMoshe, and so the construction is instead veAharon, but it means simply that Miriam spoke and Aharon was silent, or that Miriam did most of the speaking and Aharon agreed. (Indeed, see Ibn Ezra who more or less says this.)
What then of the next verse?
|ב וַיֹּאמְרוּ, הֲרַק אַךְ-בְּמֹשֶׁה דִּבֶּר יְהוָה--הֲלֹא, גַּם-בָּנוּ דִבֵּר; וַיִּשְׁמַע, יְהוָה.||2 And they said: 'Hath the LORD indeed spoken only with Moses? hath He not spoken also with us?' And the LORD heard it.--|