If you're using a Mac there isn't a system volume controller with adjustments for each application, only a single global control and software is expected to provide its own local slider. Literally every audio source I've encountered has one except for SoundCloud.
EDIT: Might be a bad example, I'm not sure if appear.in has a sound volume slider. But the point stands for every media player I've ever used, whether online or native.
True, but for those looking for a solution like this there are options. I personally use SoundControl 2[1] ($10) but there is also a free open source app[2] and a few other options out there.
[1]: https://staticz.com/soundcontrol/ [2]: https://github.com/kyleneideck/BackgroundMusic
It does, however, preclude me from turning down the Soundcloud player while keeping something else at a consistent volume.