I had this problem the first night I used my SGS6 and Spotify. The audio would go low for a while and then up loud - I couldn't get it adjusted right. I checked sound settings to see if something obvious stood out and I saw that I had checked SoundAlive+ in the Sound Quality settings. I turned it off and haven't had any issues since.
Well, one issue...my Pioneer AHV 4000NEX lost everything when I had to disconnect my battery and it took FOREVER to get my system set back up correctly so Spotify would sound good again
