probably nothing - I've not bought any music through the Google store, but it may be the case that items are in a 'hidden' directory (folder) which is why they are not visible to the Sony Walkman app, or settings (where you'd choose a different ringtone).
With luck someone who has more knowledge than me will chip in with how you can locate (and thus play) the music tracks (assuming it's possible - sometimes bought items may be encoded so copying and playing with a general purpose player won't work).
Hoping someone can help me out. New Sony xperia z, I have used the google store to download music, it shows in the play music app on the phone but not in the Sony Walkman app. It also doesn't show when in settings and sounds for call ringtones - all the tracks that were pre loaded show in all these places......... What am I doing wrong?!