The song is "Llegó el Lechero" (The Milkman Arrives) and it is blasted throughout the city by delivery trucks with giant loudspeakers selling milk, leche agria and other dairy products. You can hear the song everywhere you go in the city, and if you read the comments on the YouTube video, it's mostly from Managua residents laughing about how early it starts up in their barrio. (Supposedly the phrase has another entendre too, which is interesting.) It passes by the church office around 10 in the morning, driving slowly with the volume up to 11 and repeating on a short loop.
In general, truck-mounted (or horse-mounted) loudspeakers are pretty common, and you get everything from evangelical preachers making the rounds to ads for laundry detergent, political propaganda, scrap metal collectors and vegetable salesfolk.