There’s a saying that not all heroes wear capes. By that token, you can say that not all superstars get to the big time. Such is the case for the highly enjoyable, rousing and emotional story of Mike and Claire Sardina, a married musical duo called Lightning & Thunder. The couple are true “everyman” entertainers whose dreams and talent were never limited by their resources or their place in the world. Written and directed by Craig Brewer, Song Sung Blue gives the beloved Wisconsin-based act their big screen treatment. While they are far from the Hollywood Bowl, the very fact that this film exists is a testament to the perseverance…