I know AVAssetDownloadURLSession is for HLS content, but I tried it with remote mp3 assets and it successfully downloaded them, albeit minus the "didLoad timeRange" and "didResolve mediaSelection", and the resulting .movpkgs did indeed contain the audio data.
However, when trying to load the packages for offline playback, the asset cache reports isPlayableOffline as false.
I tried masquerading the mp3 as audio-only HLS, but during download received the error -16655. Is there a way to make download and offline play of audio-only HLS or non-HLS audio work?