1. A method comprising: receiving input media data with an input playing speed; determining one or more user identities identified based at least in part on biometric data collected from one or more users who correspond to the one or more user identities and to whom audio utterance derived from the input media data is to be played; determining a preferred rate of audio utterance based at least in part on the one or more user identities; determining a rate of audio utterance for a portion of the input media data; based at least in part on the preferred rate of audio utterance and the rate of audio utterance, generating a portion of audio output media data with an output playing speed at which audio utterance in the portion of audio output media data is rendered with the preferred rate of audio utterance; wherein the method is performed by one or more computing devices.