Most acquisitions that google goes through they shut-down, unless it's really core (like Docs). Songza will be shut down, but the team will join Play or whatever...
Docs was google's only docs service, and subsequent acquisitions have been squashed into it (e.g. etherpad). Maps is google's only maps service. Blogger was more-or-less google's only blog service until recently, and I wouldn't count on it surviving much longer in the age of google+. Youtube happened the other way around, in that google killed google video and merged it into youtube.
Is play music more established than songza? I think yes, and google is unlikely to maintain both of them in parallel.