Apparently O.Henry's A Municipal Report is some classes' homework assignment; this is the third reference to it I've seen today!
A Municipal Report is a report given by the government of a city (a 'municipality' as TrustM pointed out) that tells everyone about the city... we have xx dollars, we have xxx people, we have xxx businesses, this is what happened this year, etc.
Municipal Reports are very, very, very boring. No emotion, no... people stories.
O.Henry used that title because he, also, is making a report about a city; Nashville. (Notice how he lists boring numbers and facts several times... he wants to sound like a municipal report.)
BUT, his joke is that in HIS municipal report, even though he says a dozen times that nothing happens in Nashville, it is boring, it is old, REALLY, in his story, some very exciting things happen to the characters. He makes you expect a boring municipal report, then bam! you have all this excitement, a murder, etc.
You might do the same thing after a wonderful vacation:
Me: "What did you do on your vacation?"
You: "Nothing.. very boring... just went one or two places... very slow.... just went to Paris, New York, and Beijing!!!!!"
All this is why English learners should not read O.Henry... jes' too darn complicated...