To adjust the volume on a Google Hangout call, you'll need to adjust the volume of the device you're listening on.

Apple has discontinued the functionality of controlling the volume on the Spotify app with your iPhone’s physical volume control buttons when devices are connected through Spotify Connect, such as ...

You might have noticed that your iPhone shows a useful playback controls widget on the lock screen whenever you're listening to audio. It's multipurpose, too, since you'll see it whether you're ...