Same problem here. Weirder even is the workaround:
Moved the project folder to my C: drive, opened it with Cubase 8.5: sound working with automation.
Then I opened original project from D: drive: sound working with automation.
So simply moving that particular project to another drive, opening it in Cubase has triggered something to fix this issue.
One thing I did notice when it wasn’t working was when clicking on the fader with the mouse, sound would return while keeping the fader pressed. Weird bug with an even weirder workaround indeed. :S