I'm thinking about how the gender, talking speed and accent of the speech voice used affects user understanding. (UK market) Examples of speech interface: SatNavs, Automated Telephone Assistance Services, etc.
Somewhat old paper, but has a decent starting point: User Interface Design Guidelines for Speech
When and how to develop a speech recognition application:
10 guidelines related to the software portion of the speech recognition user interface:
However, keep in mind that natural language processing has come a long way in the past couple years making it possible to have seamless voice interaction rather than just command based voice input.