How to adjust ringer volume while listening to music?
When an app is playing sound, Android will only adjust the volume for what is actively playing the sound (in this case the media volume). You can manually adjust your sound volume under Settings -> Sound -> Volume

If your ROM supports it, you may be able to enable expanded mode for volume under Settings -> Sound -> Volume Panel Style -> Expandable. This will give you a button to the right of the onscreen volume indicator (when you press the volume buttons). Press that and it will show all the adjustable volume levels and you can change it through there.