Where does Google Play Music v5 store music locally?
My Google Music 5.3.1317M.940995 running on a Nexus 4 with CM11 stores the mp3 files under


  /data/data/com.google.android.music/files/music


You need root to access that directory.

A older version of Google Music on a Galaxy S stored it on the SD card. And, as I read in the official changelog, Google Music 5.x seems to be able to use the SD card as storage too. Which makes sense for devices with low internal storage space (/data/data).