I recently discovered this question in the closed issues of the audio_service package. I had the same question so I am reposting it here for visibility.
The audio_service package requires you to pass in a MediaItem like so:
MediaItem(
id: "https://s3.amazonaws.com/scifri-episodes/scifri20181123-episode.mp3",
album: "Science Friday",
title: "A Salute To Head-Scratching Science",
artist: "Science Friday and WNYC Studios",
duration: Duration(milliseconds: 5739820),
artUri: "https://media.wnyc.org/i/1400/1400/l/80/1/ScienceFriday_WNYCStudios_1400.jpg",
)
If you want to play a local file, though, there is no clear way to do that. How do you play a local audio file?