1. An interactive voice response system (IVR) for providing acknowledgement prompts, the system comprising: means for receiving an input audio stream over a voice channel from a user; means for analyzing elements of the input audio stream by a speech recognition system for identifying a keyword and an associated confidence value; means for determining a confidence category based on the identified confidence value; means for selecting a suitable acknowledgement prompt associated with the determined confidence category; means for analyzing elements of the input audio stream by a volume analyzer for determining a current volume; means for selecting a suitable output volume for the selected acknowledgement prompt based on the determined confidence category and the current volume; and means for outputting to the user the selected acknowledgement prompt at the selected output volume whilst continuing to receive the input audio stream.