Hi,
I’m trying to change the tempo (without changing pitch or key) of a song but I can hear digital artifacts, like garbled up instruments. It seems to be fine for voice but not on other instruments like piano… Is there a way to prevent that?
Thanks in advance.
Unfortunately the “time stretching” process always produces some artefacts, and the greater the stretch the worse it sounds.
In Audacity 1.3.13 there are two alternative effects that you can use. “Change Tempo” or “Sliding Time Scale / Pitch Shift”. The latter is able to produce the same effect as “Change Tempo” but uses a different algorithm that may give slightly better results.
Got it, thanks very much for your reply! 
I was having some issues with artifacts too, but my guess is that the artifacts are the result of successive samples having too large a difference in signal value. I was speeding up a recording by 10%, and find that if I directly apply the effect to the original 44100Hz recording, some artifacts are introduced. If I first resample the recording to 384000Hz, then apply the tempo change, and then resample again to 44100Hz, the quality is significantly improved
Other than being much slower to process I notice no difference in the final sound quality.