I’m previewing tempo changes but there is no sound unless I Apply.
It works for me with Audacity’s “Change Tempo” effect.
Which version of Audacity?
Are you using Audacity’s “Change Tempo” effect?
What settings?
Is it a mono or stereo track?
Does the track start with silence?
What’s the track sample rate and the project sample rate?
Anything else that may help us to reproduce the issue?