Loud Ringer Volume with Incoming Calls?
You can utilize an app like Tasker for this. Tasker is an automation tool, which executes configured tasks on configured associated events, which can be time-based, location-based, state-based, and event-based. So you can trigger the execution of a certain task at a given time, or when you reach a certain location, when an SMS arrives (event), a specific person calls you (event), or -- now it comes -- a certain app is running in foreground (state).

So basically, to get back to your question, you could advise Tasker: "When I'm running Spotify in the foreground, adjust ringer volume to maximum."

As a side-effect, you could also do things like "When I run maps, activate GPS"; "When I reach the office, adjust ringer, notification, etc" (to not annoy your co-workers -- does a great job for me); "When I come home, activate Wifi", and much more. Oh, even such easy things like "When I place my phone face-down: SHUT UP! All volumes to Zero! DO NOT DISTURB!" :-)

So to sum up, it's the final swiss army knife for Android...