Undo does not work on audio effects. When tweaking threshold on Duck Volume and then press cmd+Z to undo - the whole effect disappears.

This applies to all effects. Also mentioned here: https://fogbugz.unity3d.com/default.asp?619760

As a user this is a bit annoying if you for instance have tweaked a bunch of parameters and then press cmd+Z instead of cmd+S. All your work would be gone.

To reproduce:
- Open attached project and launch "test" scene
- Open Audio Mixer and select "DuckVolume" group
- Tweak some of its parameters and press cmd Z
- Observe that DuckVolume parameter disappears (because it goes back to a previous selection in editor)