When the soundcloud app is installed on my phone, the volume buttons during media playback adjust In-Call volume. It doesn't matter if the media is coming from soundcloud or not, only if the app is installed on the device. I have preformed many many many tests to verify this, all of which include signing out and clearing the cache on over ten android devices running between 7.0-current android OS.
When this app is installed on an android device, the volume rocker changes In-Call volume instead of Media volume. No matter what.