How to reproduce:
1. Open "TimelineBug" project and 'Repro" scene
2. Click on "New Timeline" GameObject in Hierarchy
3. Try to drag & drop "Audio Source" GameObject to a Bindings section of "New Timeline" (In its Inspector --> Playable Director component)

Expected result: By dragging & dropping Audio Source onto Audio Track in Bindings section you can bind an Audio Source as a Track binding
Actual result: Trying to drag & drop Audio Source onto Audio Track in Bindings section does nothing

Notes:
1. Binding an Audio Source as a Track binding was introduced in 2017.1.0b7
2. The bug does not reproduce when dragging & dropping Audio Source into the Timeline window binding slot

Reproducible with - 2017.2.0b1, 2017.1.0b7