That's not DRM. That's because the Chromecast can't store media, and requires it to be streamed. Google play music doesn't want to stream - probably because it

That's not DRM. That's because the Chromecast can't store media, and requires it to be streamed. Google play music doesn't want to stream - probably because it will eat battery, or just because they know/think that few people sideload music. You can just upload it and stream it from their servers, or find and app that will stream from your phone and accept the battery loss. I'm pretty sure these exist.