The sound may be triggered by loading the next slide rather than clicking the Next button. That next slide will have a layer holding the audio (so you won't see it on the main timeline), and triggers are set to show that layer immediately so the audio will play, then return to the base layer.
Step 1: Create a layer (I will call my example layer SoundEffect).
Step 2: Place the audio file on the SoundEffect layer. It won't hold any other objects. When you go to that layer, you should see the waveform of the audio in the layer timeline.
Step 3: Add a trigger to the SoundEffect layer so that it will disappear once the audio is finished.
Step 4: Add a trigger to the base layer that will show the audio layer SoundEffect when the timeline starts on the slide/base layer.
Check the Triggers panel for triggers similar to these.
Base layer: "Show layer SoundEffect when the timeline starts on this slide."
SoundEffect Layer: "Hide this layer when the timeline ends on this layer."
If this isn't what is happening, maybe someone else will have a suggestion.