The high-voltage line fell during celebrations in the town of Bandeira do Sul in the southwestern state of Minas Gerais, the states firefighting service said.
It said the accident may have been caused when a reveler threw a roll of tape containing metal onto the line, causing it to short-circuit and fall onto the sound truck.
Such trucks, known as Trio Eletricos Electric Trios and equipped with high-power sound systems, are commonly used to lead Carnival parties through the streets in Brazil.
The firefighters statement did not give an estimate for the number of people wounded, but media reports said 50 people had been hurt.
The cable fell on top of the truck, which then became a conductor of the electric energy and many people were electrocuted. Those who tried to get others out of the current also died, local police chief Ademir Luiz Correa was quoted as saying by the O Globo newspaper.
This years Carnival celebrations had already been hit by tragedy. A large fire three weeks ago destroyed thousands of costumes and threw the preparations of some of Rio de Janeiros top samba groups into chaos.