Checkpoint 4.4 Slow Multimedia in Digital Talking Book Specification

4.3.3. For a prerecorded audio track including audio-only presentations, 
provide at least one setting between 75% and 80% of
the original speed.

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 From the Digital Talking Book Specification ANSI/NISO Z37.86-2002

     http://www.niso.org/standards/resources/Z39-86-2002.html


15. Time-Scale Modification *
(This section is normative.)

It is strongly recommended that playback systems implement Time-Scale 
Modification
to enable user control of playback speed.  Playback rates continuously 
variable from one-third
to three times normal speed are recommended.  It is also recommended that 
players allow users the option of disabling pitch correction during TSM 
operation.

All time offsets in a Digital Talking Book (DTB) (e.g., SMIL and NCX 
clipBegin/clipEnd,
bookmark timeOffsets, etc.), are based on normal play speed. In order to 
maintain
synchronization, a player must process time offsets independently of actual 
playback speed.

...
* Time-scale modification varies playback rate (both slower and faster than 
real time) while maintaining constant pitch.

Regards/Harvey Bingham