I am using a Samsung Galaxy S6 but this also happens on an LG G4 I have used.
The Maps navigation volume is too loud over the Soundcloud app but not over the Android Music app. The navigation voice lowers my music volume to zero (or near zero) but continues playing in the background.
I am aware of the navigation volume setting and have used it to lower the volume to the "Softer" level over the Music app.
I am not sure if this is an issue with the Maps app or with Soundcloud. Any advice would be much appreciated.