Animation clip blends with Audio clip when drag & dropped on an Audio clip in Audio Track of timeline window.

Steps to repro:
1. Open MultipleClips scene from the attached project.
2. Windows > Timeline.
3. Select Cube game object.
4. Select "Recorded" animation clip, drag and drop on Audio clip in Audio Track section.

Observe that Animation Clip and Audio clip are blended together.

Reproducible on:
2018.2.0a7, 2018.2.0a6.

Working fine on:
2018.2.0a5, 2018.1.0b13, 2017.3.0f3, 2017.2.0f3, 2017.1.0f3.

Environment:
Occurring on Windows 10, Windows 7, Mac 10.13, Mac 10.12.