8. A non-transitory computer readable storage medium comprising instructions, which when executed by one or more processors cause performance of steps of: receiving, from a content server, input media data with an input normal playback speed, the input media data comprising a plurality of input media data portions each having the same input normal playback 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; receiving, from the content server, a plurality of rates of audio utterance for the plurality of input media data portions; based at least in part on the preferred rate of audio utterance and the plurality of rates of audio utterance, generating audio output media data comprising a plurality of output media data portions having at least two different output normal playback speeds but the same preferred rate of audio utterance.
12. The medium as recited in claim 8 , wherein audible pitches respectively produced by the plurality of output media data portions rendered at the at least two different output normal playback speeds and produced by the plurality of input media data portions rendered at the input normal playback speed are perceptually same.