How to control playback speed of iTunes U lectures in Mac OS?
It is currently not possible in iTunes. I highly recommend you to give feedback about this feature to Apple.

However there is a workaround:


Download VLC



  VLC is a free and open source cross-platform multimedia player and
  framework that plays most multimedia files as well as DVD, Audio CD,
  VCD, and various streaming protocols.



Right click on the file in iTunes and choose "Show in Finder"
Open the file shown in Finder with VLC
In the menu "Playback" you can find the "Playback Speed" settings.