A few months ago we told you about a nifty music streaming app called SoundSeeder, which lets you stream audio directly from one Android device to another over WiFi. At the time I mentioned that it was a cool idea, but the fact that it didn't allow users to stream Google Music songs over the connection was a bit of a let down. Developer JekApps took my complaint under advisement, and let us know that there's a new version that enables Google Play Music streaming after all. Full marks for ingenuity!
They've achieved this by spoofing the music server app as a Chromecast. So, set up your music server device with both SoundSeeder and Google Play Music, then enable Google Play Music Mode in the settings menu and enable it on the main screen. Install the SoundSeeder speaker app on one or more receiving devices on the same WiFi network, preferably plugged in to some kind of external speaker. Now you can play music in Google Play on the server device and "cast" it to the server app, which then broadcasts the streaming audio to the SoundSeeder speaker apps on the other phones. With a little setup and tinkering, you've got a cobbled-together home streaming setup with access to Google Play Music All Access.
SoundSeeder Speaker, streaming the Google Play Music track from my Nexus 5.
You can see the various components of this setup working on my own devices in these screenshots. Though it sounds a little daunting, the process itself is not that difficult, and it's surprisingly effective once everything's up and running. You can try out both the server and the speaker apps in a time-limited demo mode, and the full version of SoundSeeder is a reasonable $3.89 via in-app purchase.