1) Start Audacity and Enable the AudioUnit effects 2) Record, create or import some audio 3) Select some audio 4) Choose an AU effect 5) Play the audio and adjust the effect for an obvious change in sound 6) Click “Apply” 7) Close the effect 8) Play back the selected audio 9) Observe: no change in the selected audio. Note: usually you can see after Step 6 that the audio has not changed (for example, extreme limiting or filtering should reduce the level - or Distortion is a good use case) If you omit Step 5 the effect is applied properly.

On Mac Audio Unit effects are not applied if you initially use the preview and then try to apply the effect