Is there a music player that can play music from the Google Music Store as well as Amazon MP3?
Have you tried CloudAround yet?  It's a music app that lets you listen to everything on your device as well as sync/stream from a few cloud storage providers.  This includes offline music you saved from Amazon MP3.  The app scans your device for all music files and gives you a handy list of everything available.  Once you login with the various cloud services, your music stored will show up too.