How can I change notification volume on Android 6.0?
I'm not aware of a native solution, but my app listing for Volume control has an app called Ringer &amp; Notification Unlink. It requires root and the XPosed framework, but should do its job. From the app description:


  Since ICS, Google has decided to link the volume of ringer and notification volume together. This Xposed module can separate the two streams as they were before ICS.


I have not tried it myself and especially cannot tell whether it works fine with Marshmallow – but the changelog for the latest version explicitly mentions it should: "support for Android 6".