Running on Ubuntu 14.04.
The sound in the clip is gone when I change the playback speed. Other clips in the project do not get affected. Upon rendering the sound is not restored so it's not a preview thing.

Sound never changed and gets muted when applying the speed effect - it's a feature. You may want to work around this by changing the sound speed in some other tool (e.g. audacity) and then add it separately to your project.

Ok, thanks