I did lots of googling without finding an easy fix for my initial woes with playing music via Android Auto from my new Samsung S8.
So, I decided to try various different music players from Google Play, and found out why Pi Music player gets such a good rating. Edge music player that came with the phone is now uninstalled and so is VLC which I normally find is quite versatile (although more for videos than music)
Anyway, I have now found that with Pi installed, not only will Android Auto play music from my phone when I ask, but I can request a certain Song or certain Artist. Artist works best because then it will play all songs from that artist on repeat, whereas if you just request a song it will just repeat that one song. Great feature, I'm loving it.
Edit: I don't know if it makes much difference, but I have moved my music from the micro SD card to the phone and because it's the 64 GB model I'm having fun adding more music (was surprised how many of my favourites weren't on my phone already)