Vocative case/Talk

I'm not completely familiar with the technical jargon of verb tenses in English (sorry), but what do you mean when you say that, in English, the vocative case isn't marked? Reason I ask is because of the form "Sing in me, O muse", which is vocative. I don't believe "O" has any other function in English, so is that not a mark? -- PaulDrye