If you have two sound sources in the same application you can't do this on Windows. Music from SoundCloud and a video call on appear.in or similar, for example.

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.