Since the last "update" about 6 months ago, Duolingo puts a speaker icon on every exercise, even when there is no audio. If you view the discussion page for a particular sentence, the speaker icon is only displayed if there is audio available.
When you are doing the exercises, if you hear audio, there is audio. If you don't, there isn't. The presence of the speaker icon when you are doing exercises doesn't indicate that there is audio available for that particular sentence.