9. A computer-readable storage medium having stored thereon a computer program comprising computer program instructions, which when executed by an interactive voice response (IVR) system, causes the IVR system to carry out the acts of: receiving an input audio stream over a voice channel from a user; identifying, from the input audio stream, a keyword, a confidence value associated with the keyword, and an input volume; selecting an acknowledgement prompt and an output volume for the selected acknowledgement prompt based at least in part on the confidence value and the input volume; and outputting the selected acknowledgement prompt at the selected output volume to the user.
12. The computer-readable storage medium according to claim 9 , wherein the IVR system selects an “optional turn-take” prompt as the acknowledgement prompt if the confidence value is in a predetermined range.