There is a big problem with the way the iOS Spotify app handles volume when syncing with Chromecast devices. This has caused me to have to uninstall Spotify completely from my iPhone.
Here is what happens:
When I use Chromecast with various video applications (like Netflix, HBO, YouTube), I tend to have the volume up to max/high. That is to avoid the need to turn my receiver up too high because the sound source (video played with the Chromecast device) is too low. The problem is that Spotify sync the main media volume on my iOS device to the same as the volume on the Chromecast, even if I don't use it to play Spotify. I have experienced countless times since I got my iPhone in january that it suddenly has the media volume turned up to max. I set it down to about 30%, then the next day, if I for instance go to Youtube to play a video, the sound is on high and I almost jump in my chair when the video start. This happens even if I havent played Spotify in the meantime, it has just been running in the background.
I have also often experienced that when I actually open the Spotify app, the volume bar appear on the screen, and is then turned up to max. I have not one single time turned the volume up to max with the phone volume buttons myself, and probably 50+ times turned it down from max.
Blowing up speakers
Other than the irritation of having to turn down the volume all the time, this almost blew my B&W Zeppelin speaker. I had it playing with the Spotify Connect protocol (not Chromecast), and suddenly my phone for some reason (it was in standby mode with the screen off) syncs with Chromecast (even if I did not use Chromecast), so then Spotify turned the volume on the Zeppelin up to max, and the whole house was shaking. Luckily it did not blow the speaker, but it was close!
About a week ago, I was so tired of this problem that I uninstalled the whole Spotify app on my iPhone. I have used a few other apps over Chromecast since (Netflix, YouTube and one or two other), and not once has the volume been turned up to max on the phone.
iOS version 11.4.1 (always updated)
Spotify version (have uninstalled it so I don't know, but was always updated to last version)
Earlier this year, I contacted Spotify Cares on Messenger about this issue, explained it in detail, and they asked me to do a lot of testing. I did everything they asked, and nothing helps. And the problem remained. I now have to use my old (and EXTREMELY slow and unresponsive) Android tablet to start playing music on my Chromecast, that is very suboptimal.