I have also been having this problem where tracks mysteriously stop making any sound. Odd. I just recently got Cubase 8.5 after using 6.5 for quite a while. These bugs are why I loathe to upgrade any software that is working well. I am downloading the latest update (here for those interested http://www.steinberg.net/en/support/downloads/cubase_pro_85.html ) and hope this fixes the issue. Will report back after using the new version for a while, hopefully with good news!