אור החיים — ותדבר מרים ואהרן במשה

Q — When Miriam and Aharon spoke about Moshe concerning the Cushite woman, did they speak behind his back, or to his face?

A — Or HaChaim reads the verse precisely: it says they spoke 'במשה' — 'in/at Moshe' — not 'על משה,' 'about Moshe.' He therefore concludes they spoke in his very presence, by way of rebuke (דרך תוכחה): they had a grievance against Moshe and reproached him directly. He adds a logical proof: what would be gained by speaking ill of him merely among themselves? Their purpose was to correct him, which requires that he hear their words. This reading also illuminates verse 8's 'why were you not afraid to speak against My servant' — they had openly shamed him to his face.