Who says the following and why? I've seen you before somewhere. I haven't the ghost of a notion where; but I've heard your voice. [Drearily] It doesn't matter. You'd better sit down.

Higgins is pretending to not remember the Hills because he was rude to them before.
The Hills are remembering how rude Higgins was to them.

The Hills almost figure out where they have seen Eliza before, but don't.

Higgins has seen the Hills outside the theater but does not yet remember.