I actually double checked this today. I should disclose that I use an AppRadio 4, but I believe they should be extremely similar. I started playing music and then turned the volume to 0. Then I pressed the 'talk' button on my steering wheel to bring Siri up. I could hear her fine and was able to adjust her volume while she was talking - I noticed Siri's volume did not start at 0 as I had previously set the music to. As soon as Siri's on-screen volume display vanished I could then again adjust the music's volume, which was previously set to 0. There is no on-screen difference that indicates whether you're adjusting the media volume or Siri's volume. You just need to be situationally aware. Perhaps a firmware update could label the on-screen display more appropriately.
By the way, Maps' voice guidance volume is still controlled by the "Alerts" volume level.