Unity 5.5 has released - please note Audio breaking change
The official download link for Unity has been updated to 5.5.0f3.
Please note the release notes regarding a breaking change when using the MS HRTF Spatializer.
"Audio: If previously using a Unity Hololens technical preview release with the Microsoft HRTF spatializer and the Audio Spatializer script component, said component now only controls room size properties. Distance-based attenuation is now set up in the AudioSource script component, so the Spatial Blend property needs to be set to 1.0 and the 3D Sound Settings need to be set appropriately."
If you update a project that was begun in Unity 5.4.0-HTP, you will need to set the 3D Sound Settings | Volume Rolloff Curve for each of your spatialized AudioSource objects to Logarithmic Rolloff to get the distance attenuation to match the previous behavior.