PiTiVi core does not handle playing audio clips properly at non-native speeds.
Submitted by Brandon Lewis
In theory the only thing required to cause a clip to play back at a speed greater than or less than it's native rate is to set the media duration to something other than the duration.
To test this I created a pitivi project file (attached) with a single clip and then hand-edited the media duration of the clip such that it should play at 50% speed. While the video portion of the clip does seem to slow down, the audio continues to play back at its normal rate. There are plenty of other issues besides, but the playback issue mystifies me because a simple test script (also attached) is actually able to adjust playback speed using this method.