Despite the usage of the same ToOVRVector as GearVR, the coordinate transformation from UE4 coordinates to Oculus coordinates appears to have the z-axis reversed. This results in sounds being slightly filtered in the front vs the back. The effect is noticable with samples that have high-frequency sounds. Oculus performs a filter on sounds that are "behind" the listener, not in-front.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-25579 in the post.