Visual communication is characterized by the eye's ability to actively access parts of a two-dimensional display. The reader is active, while the display is passive. This active-passive role is reversed by the temporal nature of oral communication: information flows actively past a passive listener. This prohibits multiple views ---it is impossible to first obtain a high-level view and then ``look'' at details. These shortcomings become severe when presenting complex mathematics orally.
Audio formatting, which renders information structure in a manner attuned to an auditory display, overcomes these problems. AsTeR is interactive, and the ability to browse information structure and obtain multiple views enables active listening.
--Raman