It sounds like the audio file is indeed attached to the slide but is not showing correctly in the Timeline view.
Sometimes the audio can become 'unlinked' from the timeline view, even if it still exists in the project, causing confusion.
Try checking the properties of the slide or refreshing the project to see if the audio line returns to the Timeline.
If the audio is not functioning as expected, consider copying the audio clip to another slide and then back again as a fix.
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