AudioStreamPlayer only takes
.ogg audio files. I'm not sure how well this will work with the file format iTunes uses (songs purchased from iTunes are aac by default) . Also, not all music will likely be saved to the users disk, since a lot of music is just streamed. You could try looking into this, but I have no idea if it will be helpful: MusicKit.
Default file location for iTunes:
- Linux: not sure
Maybe something from this post would be useful in actually reading system files.