Change Tempo causes quite severe distortion of voice file. It makes the voices sound ‘gravelly’. This is noticeable from about 15% upwards, and beyond 30% it sounds awful.
The WAV file is 1411 kpbs, copied from a CD.
I also tried from an mp3 file but still the same.
Is there any way I can smooth out the distortion?

I don’t use these effects but the [u]Sliding Time Scale / Pitch Shift[/u] is supposed to give better results (and I guess you don’t have to use the “sliding” feature

If you can get-away with changing the pitch too, the Change Speed effect will give you good quality results.

In general, changing tempo & pitch independently is mathematically complex and “tricky” and you don’t always get perfect results. Melodyne & Auto-Tune can do a better job, especially if they are working on a single un-mixed voice/instrument. Those tools use additional analysis so certain sounds like “P”, “T”, and “S” are not shifted, and the basic vocal resonance isn’t changed (so it still sounds like the same person singing).

DVDdoug, yes The Sliding Time Scale plug in does a much better job - just by using Initial Tempo Change.
The sibilants seem to be increased a little but for my purpose (just speech recordings) it is of little importance.
Thanks for your help.