Complex sentence: There he discovers that the bartender, Mighty Mack (played by John Goodman) has singing talent, while getting on the bad side of the Russian mafia who have been demanding payoffs from the nightclub.
Split into simpler sentences: There he discovers that the bartender, Mighty Mack (played by John Goodman), has singing talent. He also gets on the bad side of the Russian mafia who have been demanding payoffs from the nightclub.