So as far as I can tell my audio interface is not compatible with 10.13, I had the same issues in 2 DAWs and with iTunes. My guess is that C9 should run fine if your audio interface is compatible with 10.13.
Like I said I went back to 10.12.6 and all the issues are gone.
I have to agree that if you are doing any serious production you wait until all of the tools you use are supported in 10.13. With all the plugins and interfaces it could be a while until all the bugs are worked out.
Good luck if you dive into the upgrade… Also don’t forget to backup with Timemachine so you can go back to the future!
mtr