It's rare that anything about AI's is explained - the hair thing is actually an exception. That, and abs.
But yes, it seems that older/simpler/more "robotically" styled AIs (Pintsize, punchbot, the AI therapist) have mouths that may be something like an animated screen on the face, a mobile outline of a white "void".
Although Pintsize would eat cake batter, so there must be some kind of opening, probably only available when required by the rule of funny.
Later, more humanoid AIs (the store clerks when Momo went chassis shopping, even Momo's original chassis, heck even the practice boyfriend) have mechanical/structural mouths.
Then there's Seven (I think that was her name) who has headlight for a face. Or the robotic arm from the fight club. No mouth at all!
Don't think too hard about it. There's more variety in the world of AI than humans will ever have. I was going to say, "Than humans can conceive", but you know, as AI "creators" we have conceived of all those varieties, and more we haven't even seen.
Which actually makes them a great model for a tolerant society...