System hotkey to execute cmus-remote?
I think there is a security setting in Mavericks (and possibly Mountain Lion) that causes Automator not to allow system control. See this question
Open your Console and watch the logs when you run the service. In my case, it crashes when I try to run a shell script.
I tried adding Automator to the Security and Privacy preferences, but it did not solve the issue.
As a work around, you may have to use applescript to create and app, tie that to a service, and add the app to the Privacy tab of your Security and Privacy preference pane.