1. A computer-implemented method, comprising: determining, by one or more computer processors, that a particular computing device received a message that was transmitted to the particular computing device over a network; providing, by the one or more computer processors, the message to a first application program that is associated with the message; determining, by the one or more computer processors and based on a determination of whether a context of the particular computing device satisfies a particular criterion, whether to cause the particular computing device to present an audible notification that the particular computing device received the message; causing, by the one or more computer processors and responsive to determining that the context of the particular computing device satisfies the particular criterion, the particular computing device to present the audible notification that the particular computing device received the message; identifying, by the one or more computer processors, that the particular computing device is to wait, after the particular computing device has presented the audible notification, for an audible user response to the presentation of the audible notification; starting, by the one or more computer processors and responsive to the presentation of the audible notification and having identified that the particular computer device is to wait for the audible user response to the presentation of the audible notification, a first pre-defined time period during which the particular computing device is to listen for the audible user response to the presentation of the audible notification; recording, during the first pre-defined time period, a first audible user response to the presentation of the audible notification; determining, by the one or more processors, that the first audible user response includes one or more words that identify a command for the first application program to perform in response to the presentation of the audible notification; causing, by the one or more processors, the first application program to perform the command responsive to determining that the first audible user response includes the one or more words that identify the command for the first application program to perform; determining, by the one or more computer processors, that the particular computing device received a second message that was transmitted to the particular computing device over a network; providing, by the one or more computer processors, the second message to a second application program that is associated with the second message; causing, by the one or more computer processors, the particular computing device to present a second audible notification that the particular computing device received the second message; and starting, by the one or more computer processors and responsive to the presentation of the second audible notification, a second pre-defined time period during which the particular computing device is to listen for a second audible user response to the presentation of the second audible notification, wherein attributes of the particular computing device specify that the first application program is assigned the first pre-defined time period and that the second application program is assigned the second pre-defined time period, wherein the first pre-defined time period has a different length than a length of the second pre-defined time period.