I had some of the same frustrations.  I was a zune pass customer, i purchased a lot of music through amazon when I discontinued my service last year.  I'm using an htc 8x and it seemed like nothing was working after setting up my xbox music pass.  I read a post somewhere saying to restart the phone once signed up and boom, everything was working fine.  The Soundhound app works great with xbox music app on windows phone too.  A lot like the Soundhound integration with Amazon Music store on Android.

My only frustration now is you need a PC running windows 8 to set up playlists, at least from the instructions in the app itself.  If anyone has anyway around this, would be glad to hear it.