How the speed change works
With Keep the Pitch on, the tool runs Signalsmith Stretch, a phase-vocoder time-stretch engine. Instead of cutting the audio into slices and overlapping them, it works on the frequency content of short frames, so sustained notes and voices keep their tone instead of picking up the warble of simpler methods. Every note stays in its original key; only the duration changes.
With Keep the Pitch off, the tool resamples the audio instead: a slower rate stretches every waveform out, dropping the pitch, and a faster rate compresses it, raising the pitch, exactly the way a cassette or vinyl record sounds when you change its motor speed. Use Play preview to hear either mode while you move the slider.