The essential difference between the visual and aural interface can be summarized by the following:
The visual interaction is characterized by a passive two-dimensional display which is interactively accessed by an active user. An auditory interface consists of an active aural display which scrolls constantly, and a passive listener who absorbs the information via a linear, temporal medium.As a consequence, the visual interface can rely on a fill-out form that is laid out on a two-dimensional display with the various UI elements appearing at different parts of the display. The layout does reflect the logical grouping of these elements; however since the eye can see the entire display at a glance, there is no attempt made to automatically move the focus to the logically next UI element; the user's ability to understand the underlying semantics of the dialogue represented by the form is relied on instead.
In the case of the aural interaction, it becomes cumbersome to display the entire form to the user and allow her to draw the same inferences. Thus, in a spoken interaction it not only makes sense for the UI to guide the user to the logically next UI element; it is almost essential.