Hello,
This was a Unity bug, that they fixed in Unity 5.5, so I highly recommend you update to Unity 5.5 if possible for you.
The only real alternative if you want to stick to 5.4, is to indeed comment out mixerPlayable.Destroy() but that -as you’ve seen- will not dispose the playable and (probably) create leaks.
So once again, the best solution would be to please update to Unity 5.5 and latest Slate version 1.6.5