That's odd as I have no problems at all with the AA music playback, I listen to it all the way home most evenings with no problems. Hold the button, say "Play <album>,<artist>,<track>" and it plays. When does it fail for you?
Pretty much all the time, plays a couple of songs then randomly when loading the next track or issuing a voice command the whole thing hangs. Best I've had is 5 songs I think, got hopeful an update had made it more reliable then bam it crashes again. Even worse I then can't reconnect AA as I'll get errors from Full Link "Android Auto disconnected". Only way to get it back is to task kill the AA process then reconnect.
I've tried various methods of voice control e.g. album, specific song name or just artist name. Interestingly when the voice says "Sure" it usually works; if it says "Playing Music" I know a crash is coming along shortly!
Going back to the phone there's usually a "Music Playback Error" showing in the notifications panel.
Reading around suggests it happens more with music manually copied to SD card, which is just stupid as that's where storage capacity is. I have no desire to upload all my music to Google to then pay data charges to stream it back down again, nor do I want to subscribe to their streaming service despite non stop advertising screens appearing each time the Play app gets upgraded
The Android Auto music UI also annoys me as I can't see music by artist or album. Whoever thought omitting that was a good idea needs their head examining! The iPhone music app on CarPlay blows Google away at the moment, it's the real weak spot in their offering and really lets the side down as Google Now is excellent.