When playing my created collection there is no option for Listening Skill (only Vocabulary and Speaking). But when I manage the collection, there is a button to listen to each sentence in the list. So the audio exists, but Listen mode is missing from the options. This is true in Chrome and in the app (on Samsung Galaxy Fold 3).
After changing the ‘preffered engine’ (in my phone’s text-to-speech settings) to Google (from Samsung), the Listening skill is now available for my created collection in both the app and browser.