The spoken is in the passive voice. What this allows, in english, is for the sentence to omit the person who is actually doing the verb. So your first change would be this: (I'm going to use wife) "I am already spoken for by my wife." or "My wife already speaks for me." This says that the man's wife speaks for him — she will intervene (or speak for him) if someone else tried to ask him to marry her. So: "My wife will tell you I am already married."

It is better to think of "spoken for" as an adjective. It means "taken" or "not available". This could apply to a person or a seat on the train. I could speak for a seat at this evening's performance at the ballet, but I cannot speak on behalf of the seat.