To change the Siri and turn-by-turn navigation audio levels against the main audio volume level, you need to adjust the volume during Siri or turn-by-turn playback. If this is supported you should see an alternative volume slider that, once adjusted, you can set the volume level differently to the main music volume. Another way to get the independant volume level is to use the Google Maps app. Go into settings and select the volume settings option. Whilst Google navigation is active, adjust your system volume to a suitable level to alter the volume for Siri or Turn-by-Turn audio level.