will iMovie let me add an audio track longer than 30 minutes?

I wonder if it is a limitation on the length of a single still image?

Duplicate your still image to appear 2 or three times in the timeline, select all of them & try to re-apply the background sound.

If it fails you may need to consider splitting your audio file into 2 chunks.

It's not entirely clear why you need to select all, normally you squish media in before or after a clip to add extra time to a timeline. Moving via drag in iMove can be an adventure in itself 🙂

Try selection the audio track & then hit cmd+A to select all. You may need to try other keyboard combinations to modify the selection, like cmd+click, shift+click, cmd+alt+click.

They can modify how selections are managed, adding or removing from the selection.

Try the advanced tools if they are disabled in iMovies preferences, they may not help but it is worth a try 🙂

P.S. The help menu has a few tips on working in iMovie, search in there & check the topics.

I think you need to double click the still & edit the duration 🙂

Mathematics always wins in the end 🙂

Nice work.