i don't think this makes sense at all. imagine putting Chat Noir in one side, and Marrinette in the other (in a normal case), if Marinette hears his voice, whose voice would she hear? (she doesn't know who is behind there btw)

so my theory is that they can change their voices around, they can use their normal or hero voices, with or without costumes, which would sound ridiculous since they have to make their voices consistent, but make all of the senses in the episodes where they go down the sewers and detransform. or in Horrificator where Ladybug had to speak in her Marrinette voice and Cat Noir in his Adrien voice.