Music doesn't duck/lower when played through AUX or headphones

[Itskevin12]

It's a great implentation if done correctly. Music does duck when getting a notification or driving directions through speakers, but when listening to my music in my car or while wearing headphones my music will mute (Not pause) and continue back at full volume. It's an annoyance for me since I'll be listening to music it will out of nowhere go quiet and continue. Only to notice that the reason it did that was because I got an email or text message. Please fix music ducking when listening through an aux/headphones.
My car doesn't support bluetooth so I wouldn't know if it works there, but if someone knows please drop some information as a heads up.