I have uploaded a selection of audio files via the media uploader.
I can successfully open an attachment page (single.php) for each file.
However, the 'previous_post_link()' generates the current post URL and the 'next_post_link()' doesn't do anything. They both return 'null' when doing a var_dump.
For example, I'm on an attachment page for the audio file Dusty Road, and 'previous_post_link()' puts 'http://localhost/wordpress-theme/dusty-road/' as the previous post URL. This is obviously not what I want.
I have these links working correctly on single pages for the default post type and a custom post type. Single image attachment pages successfully use 'previous_image_link()' and 'next_image_link()'.
What could be going on here?
I'm working in a child theme of Twentysixteen.