Google's voice search feature still has a long way to go. If you say "play music" or "play some music" in the Google Search app for Android, Google starts the Play Music app and opens the "I'm Feeling Lucky" radio, which is based on your listening history. I think it makes more sense to continue playing your music and use the current queue instead of creating a new playlist.
Google still doesn't support basic voice actions like "next song", "skip", "pause" or "stop the music". If you say "next song", Google only shows the search results and a list of songs from Next, an American R&B musical group.
You can say "play Alicia Keys" and Google will shuffle Alicia's songs:
You can also say "play blues" and Google will start the Blues radio. If you say "play hip hop", Google Play Music will show a list of search results for "hip hop".
That's what happens if Play Music is the default app that handles music voice actions. You can change it by saying "play Coldplay" or something similar, quickly tapping the icon next to the app's name, picking one of the music apps and tapping the selected app (I could choose between Google Play, Google Play Music, TuneIn Radio and Poweramp, but your list will be different, depending on the apps you've installed). The next time when you use a music-related voice action, the app you've picked will handle it. Most apps will show a list of search results or start playing music.
{ via +Google }